Probably the whole mixture of changes created the need to be an all surface player. I think it would center around the ATP maturing over the years. Before you had guys in different promotions and schedules were more about appearance fees. ATP now has a pretty easy to understand schedule, 4 majors, 9 Master 1000s, many 500 and 250s, a season ending tourney with the top 8.
The better they got at getting the top players to be at the same place consistently and predictably, the easier it was to get TV money. What happens with TV money, they want the most entertaining product.
It's not just a different grass used at Wimbledon that creates a bigger bounce, they use balls designed for a higher bounce to induce more rallies. The '94 final between Sampras and Ivanisevic wasn't entertaining. Watching ace, serve-return out, serve-return-volley, serve-return pass, serve-return-volley-pass can be really uninteresting if that's all the match is.
Throw in racket technology where you can turn defense into offense in one shot.
Ambition to be the best means right now you don't have a choice. Guga was a clay court specialist who was year end #1 in 2000. Besides winning the French, he went 1R, 3R, 1R in the other three majors. Obviously you aren't getting to #1 these days on that. And it seems legacy means more today to athletes than it ever has before. At least the ones who want to be the best, still plenty that are happy to make a certain amount of money and live the life.
Everything seems to just be a funnel where being good on all surfaces was the end result. That said, if Djokovic and Miurray fade away and no one else is good on all surfaces, it will be interesting if guys are as determined to be good on all if say they can just be good on hard courts and get to #1 and win the two hard court majors over and over again. How much of this boils down to Federer was able to be good at them all and Nadal wanted to climb to the top so he needed to be good at them all and then Djokovic wanted to be at the top so he had to be good at them all and Murray trying to compete does the same thing. If these guys fade away and no one takes the spot because they just became good at all the surfaces, will there be the same push from the guys behind that guy to be good at them all?