1) It's loud, a lot louder than tennis. Tennis balls have felt, and tennis racquet strings have some flexibility, whereas pickleball uses a hard paddle and an unpadded ball. People who live near courts complain about the noise from pickleball.
2) Pickleball is coming to monopolize the local courts, making them inaccessible by tennis players.
3) Pickleball involves less running and physical fitness than tennis.