The weather, the geological features, the schools, the meetups, the job opportunities... all factors.
But the biggest is that the people are truly interested in intellectual matters in a way that was not extant in the other places I've lived. My son is a programmer and nobody had a clue what he was talking about--which is fine--but they also couldn't care less to even try to understand.
And also, there is a lot of prejudice that I am glad my kids will not grow up in the milieu of.