The reason he wasn’t in the NFL anymore was the same reason a LOT of successful college QBs aren’t in the game anymore; his skillset didn’t lend itself to playing the position in the pros.
He didn’t get through his progressions quickly enough. He had a slower delivery with only average arm strength which made making some of the crucial bread & butter plays NFL QBs need that much more difficult and he wasn’t very accurate. I could go on. He simply didn’t have what it took to build a team around.
Had he converted to another position - again, as a lot of graet college QBs do - he may have extended his career because he’s such a great athlete. But, he didn’t have any interest in doing that...at the time.
In college, at most maybe you face two or three defensive players of NFL quality on any particular day, college there’s a lot more room for error. You may have your receivers able to get enough separation that even passes slightly off the mark, or late still get caught. In the NFL, you simply don’t have that margin for error.