They have served OP something they like to eat and regard as a nice meal, with no reason to think OP won't enjoy it too - a kind and hospitable thing to do for the OP.
It isn't quite to OP's taste. She's not allergic, it's not a meat pizza and she's vegan, she doesn't have a lifelong loathing of pizza and wouldn't eat if if it was the last meal in the world. She simply doesn't much like the toppings or the fact they make the base soggy.
She asks for something different, effectively (whether said out loud or not) finding fault with their hospitality. They are probably either offended or hurt or both.
Nonetheless they comply with her request, but they make a point of handing her her 'special pizza' - we don't know exactly what form this took, whether jocular or huffy - whatever, ideally they should have concealed their feelings, but OP struck the first blow here.
OP was impolite, plain and simple. If someone takes the trouble to host you and cook for you, you should eat what you are given unless it's genuinely impossible.
But we are losing sight of consideration for others' feelings nowadays, it's always 'me me me' and stuff what impact there might be on others, we daren't eat a meal that's not restaurant quality or instagram perfect, that might mean putting up with a little discomfort, and that would never do.