My guess would be that

a) building their next social network on an open platform will let antitrust regulators off their back

and/or b) a Twitter clone sounds less sexy then a web3 / decentralized fediverse play. Meta has chased every other bandwagon (metaverse, ai, etc), it's entirely possible this is just them always chasing the hot new thing so that they don't miss out. They certainly aren't going to let themselves be Blackberry and refuse to change, they'd rather desperately copy every hot new thing and change quickly to always have an offering that appeals to their customers good enough