There are probably several reasons, many not entirely clear to any one of us now, but one can guess.

I think not an insignificant reason for this is the coming expansion of the EU Digital Markets Act where Meta among a few other tech giants are labeled as gatekeepers. As always, while the EU might be one of the earlier ones, other markets will likely follow in the coming decade.

Meta will going forward be forced to open up their platforms and incorporate interoperability with other services. It starts with messages, but knowing the EU, that is probably just the first stepping stone.

If Meta have to do it anyways, they will probably want to make sure that they are the first one in establish a strong presence in the technology that every other tech giant will also need to embrace.

I don't think they care even a little about the present Fediverse community, what they do care about is the technology that Apple, Microsoft, Google, TikTok and so on will agree on to use going forward. By embracing ActivityPub early, they are betting on having already a strong position when these companies are inevitably going to have to try to agree on a common standard.