There are a lot of things that factor into when a player is drafted when they are. I feel (just an opinion) a lot of is hype from whomever be it the college coaches or even their own scouting department. The Raiders are the only team I've seen draft players more on their measurables than their football talent. Scouts also factor in the competition they faced. I've read many times where general managers have said they care quite a lot about if the player feels the game is "too big" for them. Most recently, Ted Thompson said this. If the game is "too big" for you, then it doesn't matter if you're faster than the wind because when it comes down to it, you will buckle.
Why Donald Driver lasted so late in the draft is quite simple, it's because he is white and was an Olympic talent high jumper.
The inquiry of why player X lasted until round Y is about as worthwhile as chasing ones tail. Reason I say this because if you want all those 7 rounders picked earlier, guess what, that means someone else is going to take their place and we're right back where we started.
Does the player fit your scheme?
Does the player appear to be coachable?
Does the player have weaknesses that are possible to trained out of them?
Does the player have a high ceiling?
I could go on and on, but I feel you get the drift.