It is difficult to control users, to ban them permanently etc.
It is difficult to make money with federated software. You could maybe show ads. But nothing that'd make you rich.
To be fair if nobody can get rich from it the fediverse is probably going to stay a much nicer place than the rest of the internet. Profit motives are fine when you’re dealing with strangers but they always add an element of dishonesty to communities of people you intend to stick around with in my opinion, there’s no good faith when my presence is being monetised any more than a farmer has the best interests of their livestock in mind.