Because this is reminiscent of what happened with XMPP. In the old days you had many closed source protocols for instant messaging. Then XMPP came along and started gaining steam. At that point, major platforms started using it, with everything federated. Someone with Google could talk to someone on Facebook and with someone on myown.sillyserver.net. Everything was going great. But obviously the majority of people went with the easy option to go with Facebook or Google, meaning you still had a federated network on the paper, but with a few actors weighing way more than most.

Obviously at that point, they slowly defederated, preventing their customers from talking to their contacts on other platforms. But most of their contacts where on the same platform, so the cost of migrating was higher. That's how the federation ended. XMPP still exists, and was actually used by WhatsApp in a non federated way, but it is the shell of itself with not a lot of people using it.

A social network strength is in its number. Accepting Meta into Fediverse creates a very real risk that they will try an embrace and extinguish strategy and in the end you will have most people on Meta and just a niche of people on Lemmy/Mastodon, similar to how it was a few months ago.

The goal of the fediverse is to find the proper balance between having multiple platforms big enough so that moderation and technical management can be done by knowledgeable people, but small enough that they cannot decide willy nilly to defederated. Having Meta in the fediverse would very probably break that balance.