Adding a large number of college teams would cause sponsor and venue outsourcing, and more importantly mentor outsourcing.
Speaking from the point of view of a high school student, I look up to my college-aged advisors. It's easier for me to work with a college-aged advisor than to work with an adult I don't know, and I look up to these college students because they offer advice and information that applies to me more than the advice from any other age group.
While I agree with Sanddrag that many college students need more technical experience, I'll confess that I'd be sad to see these students go. The mentors on our team offered technical training for all new advisors, regardless of their experience in FIRST. Every new technical advisor had similar training, and ended up on the same level of technical knowledge.
There are other robotics programs, but let's keep FIRST student participatino below college age.
We love you college kids.