Reason #1: It overloads them mentally. It was hard enough to wrap their minds around Mastodon not being the one website they thought it is.

Reason #2: They got so used to their nice and cosy and fluffy and friendly woolly mammoth being the entire Fediverse, and everything they ever interact with being Mastodon, that everything they (might) have to interact with that is not Mastodon is too much of a disturbance. There being something else in the Fediverse other than Mastodon simply feels too wrong.