I agree with what's been said" 1) it's enough 2) bears ain't all that tough.
I think some of it may come from they don't spend their live chased as food as many ungulates do. I think they really don't know what to do when something is trying to take their life. I've never, ever had a deer or any other non predator die as easy as my bear.
Monty, you put it tight behind the shoulder, below center of the body and it really doesn't matter what broadhead you shoot. Shot placement, as with all other critters, is key.
However, I would shoot something with a lot of blade if I could, as the fur soaking issue has been mentioned. But you put it in the right spot and blackies seem to die quick: I killed mine with an old Pearson deadhead and if fell in 5-7 yards. Course that head is like a flying meat cleaver... :D Lot of other guys in camp with good shot placement had similar very, very short trails.