I would guess it is a combination of factors. Better habitat - there are actually in many places more forested areas now than there were 60 or 70 years ago. I remember seeing a map of Ohio, and in 1900 there was almost no forest coverage across the whole state. Compare that to 2000, and there was good coverage over at least 40% of the state.
Better herd management- most state's DNR's have done a good job of managing herds.
Less hunters- there has been a steady decline in hunting for the past 20 years or more.
More land off-limits to hunting. With urban sprawl, you now have a lot of areas that still are good whitetail habitat, but are now off-limits to hunting because too many people live there.
Also, maybe some other factors I'm not aware of. In any case, I'm not complaining!