Obviously others have been in these situations and I haven't, so any argument I make here is theoretical only.
RE the finance option - I can see a time in my life, when I had just left home, where an unlooked for pregnancy would've changed the course of my life. When you make just enough money to feed yourself and have a roof over your head, having another mouth to feed would be a terrible burden. Having said that - having a baby when you I was very young would've been life changing. I am happy to say that, had I asked for support from family, it would've been forthcoming, so perhaps this is part of the reason why the financial burden is one that I see as less compelling. I am probably in a fortunate situation in this regards - my parents are not wealthy by any means, but would've, I'm sure, placed some of this financial burden on themselves.
As an aside, my parents got married because of an unexpected pregnancy - my eldest brother is extremely happy they chose the course they did
Kostya - again from a purely speculative POV, I would find giving up my child for adoption almost as hard as aborting it.
There is a legal line drawn in the sand (20 weeks through a pregnancy), prior to which is considered a miscarriage and after which it is considered a human life. My soon-to-be second child is just before that line at the moment.
I also thought of a fourth reason - not one that I would use, but... - people specifically get ultrasounds done around the 10-12 week mark looking for indicators of down syndrome and other abnormalities. We skipped this scan altogether because we would not abort in any case, but obviously people do.