A large portion of the chefs on British passenger boats to and from India were Goan, as they tended to be Catholic and thus could cook with pork beef and alcohol (the original vindaloo was cooked with pork and wine). Hence why the dish became popular. When a broader range of Indians, Pakistanis and Bangladeshis started serving Indian food in Britain, vindaloo already had an association with upper class Indian food, though they couldn't cook with pork and wine, so it got changed to an almost entirely new dish.

And it isn't very well known within India in the sense that it hasn't spread out to other parts of India outside Goa. I think western people might more readily recognise the dish than Indians lol