1. Sheer numbers - I think there are only 15 spots on an NBA basketball team while there are 80 on an NFL team. Typically, The NBA draft only produces starter quality players in the top 10-15 pick each year. In the NFL draft, it's the first 3 rounds.
2. TV time - You're right that there aren't many sleepers in the NBA draft. I think that's because in college basektball, if you're good, you're on tv. Think about guys like Stephen Curry, Jameer Nelson, and Wally Szerbiak. All played for small schools but were still household names come draft time. In football, not many people have heard of you if you don't play for a big conference school unless you're breaking records. I think that NFL talent scouts are some of the most vital positions in all of sports.