Because it costs A LOT less money to prevent unwanted pregnancies than it does to support a child.  Especially if that child has a troubled life since it was unwanted in the first place.  Poor people don't have money to pay for any of these things (birth control, health care, etc.).  So let me ask the taxpayer this: Do you want to pay Planned Parenthood for the birth control and education for the young would-be mom or would you rather pay for the actual birth in a hospital, plus the many 1000's of dollars the child's medical care will cost throughout its life.  Oh, and if they end up in the slammer due to a poor upbringing then you get to pay for that at roughly $40,000 per year.  But hey, maybe everything will break the way YOU want it to and somehow (a) the baby is born, and (b) the woman and child NEVER ONCE need federal assistance in any way.  Good luck with that scenario...
Or you could fully fund PP and save untold sums of money.  Your choice.