Of course there were a lot more faster courts, particularly carpet but it is still true that there was medium hard and heavier clay than there is now. I don't think it's all one way like people think, a lot seem to believe that it was fast year round.
Many tournaments still have similar surfaces, like Miami or Toronto.
But the fastest ATP surface in 2013 would not be top 10 fastest in 1993. That has a pretty big impact on aces. Not only does it favour aces per game played, it also means more games played per match and more matches for big servers.