Unarguably it's a global phenomenon. And personally I don't greatly value authenticity - or adherence to a particularly canonical cuisine - but changes to a cuisine's canon don't just happen due to time passed; that was really my point with the ciabatta example, that it is accepted in spite of being very recent. I haven't asked anyone explicitly about ciabatta, but I would guess that even though it happened recently, it'd be accepted by most Italians, even those who are fairly strict about what counts as authentic, as Italian cuisine.

Why, even though it's so recent? Well, in the end it's about social agreement, not rules, but there are probably some reasons that go into that agreement: it was developed in Italy; it has similarities to existing cuisine; those similarities were consciously developed; it doesn't violate any implicit expectations in the existing cuisine. Hawaiian pizza certainly doesn't share those aspects. That doesn't mean that it couldn't be accepted, but it means it's harder.

To take another example, sandwiches are an accepted part of British cuisine, and have also well and truly transcended those roots and become global. But that doesn't mean that banh mi are part of British cuisine!

Canada in 1962

And bear in mind that pizza only became international basically right after WWII, so if pineapple pizza was invented today, pizza would have only started becoming known to most in, say, North America in like 2013 or so.

It blows my mind how quickly it spread and became just something normal.

I think that's one reason I'm perfectly fine with the places that do things like put corn on it. lol

Food is meant to be enjoyed. And nourish, but hopefully pleasantly so.