Because usually that's what happens when you "debate" with vaccine sceptics: they just accuse you of being mean/pretentious/know-it-all/arrogant/etc., they move goalposts, they switch subjects, or they completely ignore you, disappear and come back later elsewhere asking the exact same "questions" as if your discussion never happened.
Because they have reached a point where hey are so much wrong that admitting it would be admitting they have been very stupid, very unjust and very bad people. And it requires a lot of backbone to own such a personal failure. So instead they double down.