"Yes, I've noticed this with several of my Cantonese chowhound friends! They're very cosmopolitan, interested in all manner of chow diversity, but the thought of eating Taiwanese, Sichuan, or Shanghai food is as unthinkable to them as the thought of eating cheese would have been to their grandparents!"
I think there are a few factors at work here. First, if one is raised eating Cantonese cuisine, the dishes of homecooking are comfort food. Then when one goes to other parts of the Chinese world and observes that the fanciest and most highly regarded restaurant in town is the Cantonese place, it does reinforce the general view that Cantonese is the best of Chinese regional cuisines. I would define Cantonese rather broadly to take in the various ethnic groups in southern China, e.g., Hakka and Chiu Chow.