It's less of a technical constraint and more of a practical one.
The servers maintained by airlines could certainly hold another year's worth of flights.
The issue is that it doesn't make much sense from a scheduling standpoint.
As it is, airlines are always adjusting capacities on routes.
They add and drop frequencies, open and close routes, and change their fleet.
Planning all of that out one year in advance takes a tremendous amount of work.
Planning out two years is even more work, but for what?
Presumably you're asking because you want to buy airline tickets earlier, but most people don't buy very early at all.
Most airline customers buy their tickets within 100 days of the flight.
Many buy within 30 days, and a huge percentage of business travelers buy within a week or two.
Hence, the airline would put lots of energy into planning a schedule and setting prices an extra year early for very little return.
That's simply not the best way to run a business, so they don't do it.
Source: Personal Experience.
I work in revenue management for an airline.
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Speaking from experience, it has a lot to do with scheduling.
I'll pick on Delta for a second.
Delta doesn't know what air routes it's going to be traveling more than a year out (they literally won't let you book beyond that).
Maybe an analysis showed a route wasn't profitable and they nixed it.
Maybe they had to add some new ones and reshuffled the schedule.
The problem is you've got a lot of moving parts.
Here's a short list of some of the considerations that go into airline schedules: airplanes pilots runway capacity (you're competing with other airlines too) gates local ground crews passenger demand for a route
In reality, schedules shift as demand does.
I've booked Delta tickets 5-6 months in advance and, in a lot of cases, I'll get an email closer to my travel that tells me my schedule has changed, sometimes by hours.
Most low cost carriers don't let you book more than 6 months in advance.
It just comes with the territory.
Incidentally, the cruise industry generally doesn't let you book more than 2 years in advance, and mostly for the same reasons.
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Aside from scheduling and related topics, calculating the cost two years in advance is hard.
Flight costs are driven heavily by fuel costs.
Predicting these in advance is hard.
As is it hard to predict personnel costs, taxes, airport charges (i.e. it's not even clear whether the city of Berlin will have a new airport in two years or not).
The margin per ticket is quite small.
British Airways earned €6 per passenger in profits in 2011.
With ticket prices in the three digits this is less than 10% margin in a highly competitive market.
A small unexpected increase in cost can make this a loss easily.
Also for a passenger it is questionable whether it is an good investment to pay money that early.
You lose flexibility in your life planning, can't invest the money elsewhere and are under the risk that the airline goes bankrupt.
travel.stackexchange.com/users/1049/johannes
Most of the GDS's built by airlines to handle their inventory and loads just do not have a year field in the date, that's why you can only book a ticket +330 days or so into the future.
I'd say this is the number one reason, with these distribution systems changing this may do to.
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As well as the difficulty scheduling in advance when you don't know what demand will be like, I imagine another concern would be fluctuation in the price of fuel over a longer period.
Airlines try to hedge their fuel costs to allow for this, but obviously the longer the period the more risk - if you're offering prices now for flights in 18 months' time, and fuel costs increase significantly in the meantime, you're facing a major financial hit.
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From a technical point of view, one could certainly design a computer system that would let customers book indefinitely far into the future.
You could say, flight 1207 flies Monday, Thursday, and Friday of every week, every year, forever.
But is the airline prepared to say that it will fly these same routes on these particular dates and times forever?
If the airline lets you buy a ticket for 20 years from now, they would be committing that they will still be offering that flight in 20 years.
What if market demands change and they want to reduce the number of flights?
Or just reshuffle them some, like make a flight leave a little earlier or later.
The airline can't commit to never, ever dropping or changing a flight.
So they have to put some realistic limit on how long they will commit to this schedule.
Someone else on here mentioned that "most" customers book within 100 days of the flight.
I haven't seen statistics, but the number of people who know they want to make a certain trip more than a year in advance has got to be very small.
travel.stackexchange.com/users/2778/Mark Daniel Johansen
Why it is not possible to book  airline tickets  more than 12 months in advance? Why there is always 1 year and not 2 years or more? Is there any technical aspect behind this policy?
Why can't airline tickets be booked far into the future?
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<airlines><bookings>
travel.stackexchange.com/questions/112427
travel.stackexchange.com/users/75577/N Randhawa
It doesn't make a lot of sense for a passenger to book that far in advance, and setting this limit allows the airline to reschedule or drop a flight as well as changing the price. Issues with scheduling so far in advanced is one of the main issues with allowing flights to be booked so far in advance. Aside from that, from a business stand point it is just not a smart idea to allow customers to book so far in advanced as airlines could not calculate the actual cost of each flight, would not be able to commit to each flight etc.
One issue is that it does not make sense from a scheduling stand point.Planning out a whole years worth of flights takes a tremendous amount of work and airlines do not know what routes they will be flying passed a years time. From a business stand point it is not the smartest move as there are a number of variables that cannot be accounted for if flights are made available more than a year in advanced such as; Cost of flights, whether or not they can commit to the flight and whether or not it would be a smart investment from the perspective of the client to book over a year in advance.