It was a number of things that added up... On the field, he has really bad mechanics and footwork. He takes too many sacks and forces throws into coverage. Forcing throws often times worked out for him at Colorado, but won't in the NFL. But the big problem(s) were in other areas... First off, he didn't hire an agent and made terrible decisions because of it. For starters, he committed to playing in the Shrine Bowl, which now rivals the Sr. Bowl in terms of its importance to the draft. Instead of playing, he backed out and showed up to do an interview instead. That rubbed a lot of teams the wrong way. Then at the combine, he again elected to not do the workouts. That was a might bold, and stupid, move for a guy who was fighting to regain lost draft stock. He did interviews though, which didn't serve him well... again, causing teams to drop him on thero boards. Finally, at this pro day, he was sloppy and didn't even finish some of the drills. Yet another unforced error and terrible look for teams to digest. On top of all of that, his helicopter dad loomed and basically admitted that he will be a distraction to whatever team chose his kid if they didn't use him correctly. And when your dad is Deion Sanders who can generate headlines anytime he speaks, not some average Joe who doesn't have an amplified voice, it matters. A lot. NFL coaches and front offices don't like outside distractions.
So basically, it was a number of issues and bad decisions that snowballed over the last year or so.