Re: Why the decline in the number of 3+ slam seasons.
For one thing, it is difficult, given the depth and talent that exists in the WTA. Getting through 7 rounds at a slam has become a bit more difficult than years past. Another reason, is that today's game has become more mentally and physically demanding than years past. Coupled along with all the obligations players have to fulfill both on and off the court, it is all the more difficult for the majority of players to bring a level of focus and consistency that's needed to win one slam, let alone three or four. Injuries also play a role. Most players, especially the ones at the top, have not been able to stay injury free, year-in, year-out. Having to play so many matches (especially on hard courts), along with the increase in having to travel so much, also takes its toll on all the players. Obvious reasons to this, is despite the depth and talent the WTA had and still has, that very FEW players have the heart, determination, and hunger it takes to win at the very highest level. Very few players have the right tools needed to win slams. Most players can't stay consistent enough and don't know how to "peak" when it comes to big events.
Take Petra K. for example. Here's a player who we all know has the game to be dominant. She's won 2 slams and is plenty capable of winning more. Her problem, she doesn't seem to want to fully embrace her talent or embrace being at the top of Women's tennis. She is every bit as capable as Serena and Maria of winning multiple slams in a season. But as long as she continues to settle for basic or mediocre greatness, she probably never will.
A few reasons why Serena was able to do this, was because she was the only player who truly wanted to win bad enough. She remained healthy, although not at 100%. She embraces the talent she has, and is willing to do what it takes to translate that into success. Serena wants to be at the top. She wants to be a success, and settles for nothing less. Serena works hard, win or lose, she doesn't let it stop her. If it weren't for so many injuries, issues and distractions in years past, Serena definitely could've had a couple more 3+ slam seasons.