The fans base is made up of current students, alumni, and regular people. You need to remember that many states do not have professional sports teams so they hang their hat on College teams. From what I can gather it is way more popular in the south- particularly those states with teams in the SEC conference (Georgia, Alabama, Mississippi, Kentuky etc.). Those supporters are very parochial and passionate about it with many of them not caring at all about the professional teams. There was a thread on a Georgia board I frequently read about why people care more about college football than the NFL and a lot if it came down to loyalty of players, no affinity to any NFL team, money thrown around by teams to recruit players, ego and poor behaviour of NFL players, and the general product of the NFL being inferior to what is on offer at College level. Of course there are other factors but those are the main ones IIRC.
As a fan of both College Football and the NFL, I think College is a much better and more enjoyable spectacle than the NFL.