As some have said, it's not relevant (we hope) anymore because this is a new set of decision makers. But, as with most things in the NFL and life, it has historically been a combination of many factors. Put them in whatever order you think is most appropriate:
1) Inability to evaluate who will fit the scheme consistently
2) Heavily intertwined with the above, a scheme that is both mentally taxing (even on veterans) and not commonly run in the college ranks, making it more difficult to scout (and a harsher college to NFL transition for the player).
3) Not hammering the position with consistent high picks, thus leaving success/failure to a handful of dice rolls.
4) A philosophy that makes it harder for young players to get snaps unless they're already better than a veteran (which has its perks for competitions sake, but makes player development in the long term a bit harder).
5) Bad luck (not my heaviest factor, but guys like Mitchell showing promise but never being healthy was a killer).