to the major leagues. NFL draft is much smaller ( seven rounds), the players have been playing at a higher level (college), and the NFL teams have probably done a lot more evaluation of an individual player thru individual workouts, combines, etc.

You could say that MLB drafts players as a commodity, and develops them in their minor league system, while NFL drafts already developed players. So each NFL draft pick is much more valuable compared to a MLB draft pick. Much tougher to recover from a player changing his mind.