I think it's a combination of a lot of things:
1. Players are stronger and more athletic than ever, and they don't necessarily need to volley every point.
2. The racquets and strings these days allow for even more power, which translates into baseline tennis.
3. The grass at Wimbledon itself has changed a lot over the years. Patrick McEnroe was talking about when he played (early 90's I believe) how you basically had to throw the ball against the ground to bounce it back up just to serve. He compared that to today and said that you can basically just drop the ball and it will come back up to hip height. Now I'm sure he is exaggerating somewhat, but they do use different seeds nowadays.