I think it just has become more disciplined, and teams now are structured in a way that they will support the sprinter, the climber, the gc contender, whereas years ago it was mostly about supporting the team leader...
I think also, simply the schedule is tougher, not that there are more races, but that there are more races that receive media attention, so the teams want to do well in all of them, and as such have specialists in certain areas.. Could Merckx be beaten in the TDF nowadays.. Probably if there was a rider who focused purely on the TDF GC with his entire team behind him.. Could Cippo be beaten, probably if there was a team willing to focus its efforts purely on beating Cippo.. The focus now seems to be it is better to have a few riders who excell in something, than a few riders that are good at everything..
Take for example Lance or Contador (as they seem the easiest), both can climb, both can time trial with the best in the world.. Can LA sprint? Hes probably got a half decent turn of speed on him. Enough to get top 10 placings regularly, probably. Enough that in something like the giro where there are points for sprint and climbing combined.. Probably.. Would working more on his sprinting, contesting the sprints for the points have made an difference.. Probably, I doubt the extra efforts involved would have led to him winning 7 tours.. So he concentrated on what he wanted to do which was winning tours.. Sean probably wasnt goog enough to win the tour, but probably also not good enough to be the worlds best sprinter, just a tiny bit short of being the best at something.. So the best at being nearly good enough.. (thats not being harsh.. i loved the guy)
edit: of the current crop EbH has the potential