Because the internet is not about twitter and facebook, as much as they might prefer it that way.
Twitter doesn’t give a shit about section 230. If it’s repealed they’ll just engineer a “censor switch” that anyone can pull in the same way folks do today with youtube. They’ll be just fine. Anyone who’s not twitter on the other hand will not be able to afford to employ such measures and will disappear.
Removing section 230 without a replacement will simply make the twitters and facebooks of the world stronger.