I think his dad got his evaluation wrong. That was the very first problem. Also, when considering guys who are just average in talent, such as Mac Jones, Kenny Pickett, or guys who measurables, but watching the tape leaves you questioning like Jones/Haskins, NFL factors that in.
Teams want more than Jones/Pickett when they need a QB in RD 1, unless you are a QB guru that can coach and scheme up the QB.
Then his attitude in some meetings. If you aren't going to put your best effort, why take the meeting? Carter/Hunter didn't really have to meet with anyone past NE or JAX at the most. Any team can say they want to trade up, but it does get more difficult the further back a team is. Sanders wasn't that kind of talent, but since he thought he was, why take those interviews vs just the ones you want to do. Doing that would still pull him off some draft boards.
No team blackballs elite talent. Sanders just didn't have that.
Eli, had similar measurables to Peyton, plus Peyton was in the NFL for 6 years and performed great. Teams wanted their Peyton, so no way teams were going to pass on Eli or not trade for him.