In 2007, Novak Djokovic entered the top 3 in the world.  The current top 3 are Novak Djokovic, Rafael Nadal, and Roger Federer.  This is not normal (or was not normal).  So how did we get here?  I’ll take my best shot at answering.

1.  Transcendent Talents – While Federer, Nadal, and Djokovic have a different set of gifts, each is preternaturally gifted as a tennis player.  Rafa boasts a combination of strength and speed few have seen.  Novak is the best return man in history.  Federer routinely is among the ATP’s top 5 in serving points won despite not having an overwhelmingly fast serve.  Nadal and Federer have different but dominant forehands.  Djokovic’s backhand is elegantly efficient and lethal.  In short, these three are super talented.
2. Masters of the Kinetic Chain –  Tim Mayotte has led the way in researching how players can use energy from moving to the ball to also strike the ball and to use energy used striking the ball for also moving toward the next shot.  There may be faster sprinters from point A to point B on tour, but the big 3 are from Mayotte’s perspective demonstratively better at transitioning from hitting to moving and transitioning from hitting to moving than everyone else on the tour.  This may win them a few points every set that other points would otherwise lose.  Add that up over 3 of 5 sets (or 2 of 3 sets) and … you get lots of wins.  This could fall under the preternaturally talented category, but I think it deserves a category all of its own.  It may be THE reason they are so dominant, so maybe Mastering the Kinetic Chain is reason 1A for their dominance.
3. Surfaces Play More Homogeneously – Clay courts today typically play a bit faster than they used to.  Watering the courts between sets is not longer done.  Wimbledon consciously set in motion plans to slow the courts down after the 1998 event.  Perhaps, the semifinal serve-fest between Richard Krajicek and Goran Ivanisevic won by Goran 15-13 in the 5th set was the deciding factor.  Perhaps, Sampras defeating Ivanisevic in 5 sets with few rallies in the final did it.  At any rate, grass has slowed down and supreme courts (carpet) are no longer used on the ATP.  If everything plays at a somewhat similar pace with somewhat similar bounces, the best players should generally win.  Volleying to keep the ball from bad bounces on grass is no longer the norm and neither is clay a complete test of fortitude.
4. New Stringing Techniques and Technologies – Producing massive spin became easier in the late 1990s.  Guga Kuerten used polyester strings and came out of seemingly nowhere to win the 1997 French Open.  Guga also seemingly glided across the court but a hip injury and far longer strokes probably precluded him from piling up majors.  Djokovic and Nadal grew up as juniors using these strings.  Federer was able to shift to these strings in his early days on tour.
5. Money – Endorsement money and prize money allows top players to take some time off while also growing in wealth.  Roger Federer skipped the second half of 2016 only to real off titles in Melbourne, Indian Wells, and Miami to start 2017.  Rafa has taken his share of time off, including missing majors, for the better part of a decade.  Playing hurt is not imperative.   Similarly, travel options such as private jets and sports medicine advances are at the fingertips of the big 3.  Not needing to aggravate injuries by playing when hurt, being able to travel with some comfort, and being able to prevent/treat injuries with the best sports medicine has to offer all contribute to the remarkable longevity of the Big 3 along with other players doing damage into their mid 30s.