Short answer: money.
Long answer: I had an ACT in the 30s and a GPA of 3.9 upon graduation from highschool, and I took a few AP courses (my school only offered 3, and in addition I took AP Physics over the internet (terrible idea)), got a 5 in the AP Calc test, etc. but past that I hadn't taken the SAT subject tests or anything, so I wasn't really competitive. My family and I don't have a lot of excess money just lying around, and basically my options were thus: get admitted to a couple upper-tier-but-not-Ivy-League schools and pay with money I didn't have, or go to a crappy state school with scholarships.
I've had a job since 7th grade (but there are not a lot of tech companies in my area, so they haven't really been 'tech' jobs), but all of that was saved for buying my own vehicle, insurance, 'spare' money for college, etc. I remember getting excited when I received a ~$20K a year scholarship to a "Blah Blah Institute of Technology", only to find that it cost $50K a year to attend.
Overall, I'm burnt on the education system. My goal now is to get out with a stupid degree before I completely crash, but I'm already coming dangerously close...
EDIT: I was also in Duke's TIP program, which got me mail from Columbia and the like, but once again I couldn't really afford to go through with any of that.
EDIT 2: Before this sounds like just a bunch of whining, a huge chunk of this was my fault. I could have had higher scores and grades and whatnot, but I really just didn't give a shit. I hated school, and wanted to do as little as possible. I've always loved learning though.