Because in the past, the majority of updates have been implemented by the Bitcoin Core team who are a team of trusted and experienced Bitcoin developers. SegWit2x has been proposed by a different team altogether, some miners don't trust them and would rather stick to a team with a proven track record (Bitcoin core and their BIP 148 UASF). Also, for whatever reason, Bitcoin Core arent on board with SegWit2x, if they do end up supporting it all of the hash power would support SegWit2x and there would no longer be a problem.