RE:
It isn't just Sampras. Take a look at Stefan Edberg, for instance. 1990 stands out in particular - he lost in the AO Final, won Wimbledon, and went out in the 1R of both the French Open and US Open. Boris Becker is similar - lots of early round exits.
Now Jimmy Connors and Bjorn Borg were more similar to recent players in terms of consistent dominance (making it to the QF or later in Grand Slams), at least during the 70s when both were in their primes. Connors had 27 QF Slam streak from 1973 to 1983, with 21 of those 27 being SF or later. After losing to Kevin Curren in the 4R at the '83 Wimbledon he started another shorter streak: seven straight SF or later Slams. So from Wimbledon 1973 to the end of 1985 Jimmy Connors only went out once in a Slam before the QF, an amazing feat - and similar to Roger Federer's span over the last decade.
So it is interesting to note that the two greatest players of the 70s, Connors and Borg, were similarly dominant as the greatest players of the 21st century - Federer, Nadal, and Djokovic. But it is the great players of the 80s and 90s - Wilander, McEnroe, Becker, Edberg, Sampras, and Agassi - who were more erratic. The lone exception is Ivan Lendl, who is in some ways a bridge from the 70s and 80s (along with McEnroe).
The point being, for whatever reason the 80s and 90s saw more erratic performances from great players. This leads me to believe that it had more to do with the tennis of the time than it did the quality of players. Certainly the late 80s to early 90s saw more all-time greats playing at once. But I'm wondering if the courts were more diverse during that time; as some have mentioned, over the last five years or so the courts have become increasingly uniform - were they similar in the 70s and early 80s? Was it just the mid-80s to early-00s that saw a wide diversity of court types and speeds?