It's because it was not a thing in Italy, so accepting it was seen as the sign of being out of touch with "real pizza" traditions. Sadly, a natural reaction of disgusted rejection by Italians has been adopted by snobs all over the world.
Nowadays you'll find it even in Italy, because tourists demand it and they pay hard cash, so fuck it all anyway.
Nowadays you'll find it even in Italy, because tourists demand it and they pay hard cash, so fuck it all anyway.