Winning 3 of 4 slams in a single season:
Jimmy Connors did it in 1974
Mats Wilander did it in 1988
Roger Federer did it in 2004, 2006 & 2007
Rafael Nadal did it in 2010
Novak Djokovic did it in 2011
Seems like this is becoming more commonplace. Why do you think this is?
Tennis Abides: Dan Martin on Tennis and Life 
If they are just that good, why did Sampras never do this. He was that good. Becker never won a clay court event. Agassi never did it. Both guys were reigning at 3 slams in a 52 week period, but neither did it January – September of the same year. I lean toward all the surfaces playing more or less the same so the #1 player should be favored at 2 or 3 slams and be among the top contenders on the 1 or 2 events slightly outside of his favored court speed.
I sense a Peter Bodoesque nostalgia for the big servers of the 1990s from tennis fan.
I think it’s a combination of the surfaces being more similar and the dominance of the top three. I think they have beaten down the rest of the men’s tour so that there is a lot less belief in players that they can pull of a big upset.
Think its primarily the all court game dictating.There are certainly fewer baseline specialists and even fewer serve-volley exponents these days.You might be able to serve out of the proverbial tree but you were probably coached too not lean to heavily on that aspect,which probably says volumes for coaching standards too come to think of it!Conversly a baseliner can still be a baseliner as long as he can volley well enough and that has happened too.