A lot of it is because he was doing introspective relationship movies with elements of self-depecating comedy at a point where nobody else was doing it, and using somewhat of a singular eye when it came to directing, framing the shot, use of music, and naturalistic dialogue. But loads of people do the same thing these days, and Allen’s best movies are 40 years old now. Cinema is hugely different now, and comedy and relationship tropes age very badly, so the social impact of Allen’s work is more in its descendants rather than maybe going back and watching them now. It’s like how Citizen Kane did so many tricks first, but later movies arguably did them better.
And Manhattan is just horribly creepy