Mai Shiranui will not be joining Super Smash Bros. Ultimate because the game is "for good boys and girls of many different ages"

I should mention that it was CERO that blocked Mai's inclusion in Smash. You might not have heard it clearly in the US broadcast, but you can hear Sakurai mention CERO in his native language. Sakurai has complained about CERO censoring content before in regards to 4.

Even if CERO wasn't strict, the ESRB would have cracked down anyways, so Sakurai couldn't win in either case, so there was no choice but to omit Mai even if he considered using Mai's Fatal Fury outfit instead of her King of Fighters outfit.