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.
Basically, a Soft Fork is a software upgrade that is backwards compatible whereas a Hard Fork isn't. This means that people who don't upgrade when a Hard Fork is implemented will be kicked off the network, while people who dont upgrade when a soft fork is released can stay on the network but they wont be able to see/use the features implemented in the Soft Fork. With Steem a Hard Fork was implemented because it would be messy if there were some users earning more than others. With the BIP 148 Soft Fork, once the initial chain-split resolves, users who havent activated SegWit can still remain on the network and detect new blocks. However, their transaction speeds will basically be slower. But for the chain-split to resolve, the majority of users must accept the Soft Fork. Therefore, only a minority of users will be using old software.
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.
Ah, gotcha. Why did they go for a soft fork rather than a hard one like Steemit did, by the way?
Basically, a Soft Fork is a software upgrade that is backwards compatible whereas a Hard Fork isn't. This means that people who don't upgrade when a Hard Fork is implemented will be kicked off the network, while people who dont upgrade when a soft fork is released can stay on the network but they wont be able to see/use the features implemented in the Soft Fork. With Steem a Hard Fork was implemented because it would be messy if there were some users earning more than others. With the BIP 148 Soft Fork, once the initial chain-split resolves, users who havent activated SegWit can still remain on the network and detect new blocks. However, their transaction speeds will basically be slower. But for the chain-split to resolve, the majority of users must accept the Soft Fork. Therefore, only a minority of users will be using old software.