Because, despite us being free to pass judgment on the man who commits infanticide - the killing of children - abortion is basically the same thing. Do a Google Images search on "aborted babies". Tell me you're not absolutely horrified by those pictures. Doesn't that look like murder to you? Killing a human being before it can even begin to fathom what's going on around it? Abortion is in this way the most hypocritical thing that a person can do.
It doesn't actually matter if you're really young and pregnant. The child is your responsibility. Accept and perform your duty and don't run away from it by murdering the most innocent thing in the world. (I'm not referring to you here, I'm speaking generally.) My girlfriend's pregnant (we're both pretty young, I'm 16 and she's 21) and we're definitely not aborting our little girl.