You would think so, but I think that they think they have enough hash power to take over the whole thing. They may be right. They are clearly making a power grab and opposing Segwit2X: https://blog.bitmain.com/en/uahf-contingency-plan-uasf-bip14...

Segwit2X would activate effectively Bip148, though it was originally a proposal to use a different mechanism it is acceptable enough that Core is happy to have segwit activated that way.