1x9 setups are more common on a mountain bike or commuter bike setup with rapidfire shifters than a drop bar setup.
However, due to a lack of front derailleur they can have chain jump issues due to the effect of the rear derailleur on the chainline.
This has to be compensated for, often with a chain guard on the outer side and jump stop on the inner side of the front chainring.
It's also a bit weirder to have 2 STI brifters and only have one of them for shifting on a road / cross bike.
Especially when brifters cost a lot of money.
Most groupsets come with all the drivetrain and shifting gear for a 2x9 or 2x10 setup.
So, they install it on the bikes as a set, It would probably be cost prohibitive for manufacturers to break up the groupset and they would probably have to pay for extra parts (like a chain guard) to make it work.
There are however several bikes on the market with a single front chainring and an internally geared rear hub.
These bikes don't suffer the downfalls of having to deal with chain jumps as much as bikes with derailleurs since the chainline stays consistent (and hopefully releativly straight).
UPDATE:
Modern mountain bikes and some cyclocross bikes have been switching to 1x drivetrains as a standard drivetrain option.
New technology, clutch rear derailleurs, narrow wide chainrings (there are several variations), and wider range rear cassettes (up to 44t) have made it so you can have a wider gear range and get rid of the extra chain retention gear that you used to need for this setup.
SRAM and Shimano both sell 1x drivetrain groupsets and now some frame manufacturers are dropping support for front derailleurs.
bicycles.stackexchange.com/users/2997/Benzo
There are plenty of bikes available with a single chainring.
However, only certain types of bicycles come with single chainrings.
People generally buy according to their needs, and products are generally offered in accordance with demand.
The two main categories of bikes that often come with a single chainring are comfort/city bikes and track/single-speed bikes.
Comfort bikes such as beach cruisers often have coaster brakes, chain guards, and other features to minimize the amount of maintenance required.
Track bikes were originally designed for racing at velodromes, but they have developed a broader appeal due to trendiness and low maintenance requirements.
On the other hand, road bikes and mountain bikes generally have a wide range of gears to allow a wide range of speeds.
If they came with only a single chainring, they would not meet the needs of most cyclists, so they wouldn't be bought.
In short, there are plenty of bicycles with single chainrings, and they tend to be bikes that are designed to be low maintenance.
Bikes designed for performance usually come with a wide range of gears.
bicycles.stackexchange.com/users/3649/amcnabb
I suspect marketing.
More is always better.
If you were naive to bikes and you saw two derailleur bikes for $499: One with 9 gears (1 x 9) or one with 27 gears (3 x 9).
Which do you think they will choose?
In western society, more is always and without question better.
The single chainring bike might have better components (due to being simpler at the same price point), but people will fixate on the number of gears.
That said, I have noticed quite a few 8 speed hub internal hub bikes on the market now.
Single chain ring with essentially the same number of gears as a single chainring derailleur bike, but they can do a marketing end run around the issue of number of gears with a different sell, namely the reduced maintenance and simplicity associated with an internal hub.
Aside - I will avoid any discussion of whether or not internal hubs are simpler and lower maintenance, just that this is the common "knowledge" about internal hub bikes.
UPDATE
well the market forces have swung and now many high end bikes are switching to 1x (11 to 13 speed cassettes make this feasible), so more 1x are now finding their way to lower end bikes.
bicycles.stackexchange.com/users/4534/Rider_X
A wide range of gears on the rear is 11 T to 34 T which lets you vary your mechanical advantage by about 310% on a bike with only a rear derailleur .
The front derailleur is (compared to the rear derailleur) a simple piece of equipment that greatly increases your ability to go fast on the flats while climbing comfortably.
Adding a front crankset that goes from 24 T to 52 T lets you vary your mechanical advantage by ((52/11) / (24/34)) or 670% with both derailleurs between large-front/small-rear and small-front/large-rear.
(The actual mechanical advantage has to take into account the ratio between the pedal circle circumference and the drive-wheel circumference, but I'm dividing mechanical advantages here so that constant divides out.)
Bikes with only a front derailleur aren't made today because they don't pick up the slack in the chain, though I've heard that early model chained bikes were often ridden with slack chains.
bicycles.stackexchange.com/users/4126/Mike Samuel
In case of road bikes, back in 2012 (when question was asked), there was simply no option.
There was some cyclocross setup but didn't really sell in this market.
Road single chainring group set only available in 2015 by SRAM.
I just switched to their 1x11 (48t 11-36) from 2x10 (50/36 11-26).
Hardly miss the closer ratio of the 2x10 or top speed, which probably would be important if you race.
I really appreciate the easier effort needed to clean bottom bracket area.
I can see that 1x12 setup is coming, so expect to see more people switch from double chainrings.
And then we'll probably can have single chainring specific frames, i.e. without the extra cable stops, hole and routing for front derailleur.
bicycles.stackexchange.com/users/3042/imel96
Bikes with two or three chainrings are the norm, and then there are singlespeeds, but except for kids' bikes, you see almost no stock bikes with a single chainring in front but a cassette and derailleur in the back. It seems like for a lot of people this would be a nice balance with respect to simplicity, weight, maintenance, so why are there so few?
Why are there almost no bikes with a single chainring?
bicycles.stackexchange.com
<gears><chainring><drivetrain>
bicycles.stackexchange.com/questions/11114
bicycles.stackexchange.com/users/4825/Joe Ganley
There are plenty of bikes available with a single chainring.  However, only certain types of bicycles come with single chainrings. The two main categories of bikes that often come with a single chainring are comfort/city bikes and track/single-speed bikes. Comfort bikes such as beach cruisers often have coaster brakes, chain guards, and other features to minimize the amount of maintenance required.  On the other hand, road bikes and mountain bikes generally have a wide range of gears to allow a wide range of speeds. However, more bikes are appearing on the market with a single chain ring and they are generally being marketed for the reduced maintenance and simplicity associated with an internal hub.   Setups without front derailleurs are common on bikes designed to be low maintenance, especially with hub gears. On performance bikes, the second gear greatly increases the range and reduces the angle between the front and back chainring on high and low gears. However, new technology allows a wider range and more chainrings, allowing manufacturers to use a single front chainring. While many people assume that more gears are automatically better, the popularity of hub gears in city bikes and the introduction of 11 to 13 speed cassettes is changing the market in this regard.
Setups without front derailleurs are common on bikes designed to be low maintenance, especially with hub gears. On performance bikes, the second gear greatly increases the range and reduces the angle between the front and back chainring on high and low gears. However, new technology allows a wider range and more chainrings, allowing manufacturers to use a single front chainring. While many people assume that more gears are automatically better, the popularity of hub gears in city bikes and the introduction of 11 to 13 speed cassettes is changing the market in this regard.