Easy. Because Twitter is their biggest closest thing to a competitor. And right now under the shepherding of the petty little man child is floundering. The enemy of my enemy is not my friend. But they sure can be useful.
Threads launched to the audacious soft squishy thud of a freshly fallen turd. Millions of potential users who don't give a shit about it. On the other hand. Feddiverse users though fewer are wildly, passionate and engaged. So much so that people on a largely disconnected feddiverse system are losing their ever-loving minds about meta even coming anywhere near them.
Right now, realistically we're nothing to reddit or Twitter. I love the feddiverse. I'm a jabber/XMPP advocate since the 1990s. But let's be honest, we're still a pretty small group compared to social media over all. Meta however thinks it's worth while to form a coalition to topple the twit. That it's worth while to them to tolerate to some extent a den of lefties, Marxist, and even murderous leninists that couldn't be more anti them. I'm with them as long as it takes to topple musk. And then we're coming for their user base. To Free them from their algorithms and pro-corporitist censorship. Coalitions go both ways.