How can it be easy to switch from hard courts to clay to a couple of weeks on grass? Sorry but your statement doesn't make sense at all. The surfaces are the same for everybody and they adopt. Now everybody can play at least decently on clay and hard courts because they train on these and play a lot. And they mentally commit to the longer rallies that clay demands. Hard court tournaments are half of the season, that is the choice of governing body of professional tennis. So they mostly train on these courts. There is also enough of clay courts, enough for players to train on them and have success on them. Why is it today's players fault that people in the past didn't want to bother with clay if they were good only on fast courts? Or didn't bother going to Australia to play that slam that wasn't as popular as today?
On the contrary, it is harder now because the technology allows a lot of players, who want to train hard, to achieve decent results on almost every surface that is being used today. Before you had more specialists on specific surfaces, but now everybody trains to play well on every one of them. Some more successfully than others, but nevertheless. Some of the Spanish or South American players, who are considered clay court specialists, might have won more on clay in the 90s than what they are winning now.
Grass is the only surface that suffered the modernization of tennis game, going from 3 majors on that surface to only 1 and only about 1 month of grass season. If there were more tournaments on grass, players would have trained harder and more often for them and they would have been more comfortable on grass, I am sure.