Nfl is different because of the physicality of the sport.
Not really.
If the NFL doesn't think the guys are ready, they shouldn't draft them. Or they should take the responsibility of sitting on these guys for two or three years and letting them develop. Or they should build their own minor-league system.
That's actually why the rule will never change: the colleges don't want it, and even if the NFL front-offices wish the occasional Clowney could just leave when ready, they don't want either the responsibility or the expense of developing these guys. They want the colleges to do it.
Just look at the reason the NBA has the rule in the first place. It was framed as "helping" the sport of college basketball (because they NEED a lot of help), but if the NBA really wanted the increased risk of those high-school guys they would have kept the rule as-is.
It sucks for the players, who should get to go when they want, but until something fundamental changes within college athletics you won't see a significant change.