Few reasons off the top of my head:
- Slower courts are giving the returner more time to make contact with the ball after the bounce.
- Racquet and strings technology makes it easy for players to rip passing shots, even off of returns.
- Add all of those points together, the server is still in no-man's and the return is either zipping past him/her, or dropping in front of him/her with some absurd angle.
Wilson BLX Six.One Tour 90. 374g, 8pts HL, SW=355 (according to TW's calculator)