I think they appealed to the general public, as well. I know at first it was all about how long their hair was and the fact that they were from up north but once they had been in the public eye for a bit, and most in the UK knew of them, people got to see how charming these four lads were. I know they're not all the same but if you look at the majority of artists nowadays, a lot of them haven't got that (for whatever reason).
That's a contributing factor as well, I think.