Because none of those other people want to. The only reason they even allow Bernie to do as much as he does is to placate progressive liberals, if they didn’t folks might start figuring out that reform doesn’t work.
If Bernie fucking Sanders is too far left for you then you’re definitely part of the problem
Just because they’re polite doesn’t mean they’re not wrong, and if they can’t handle being told so then your talking with them isn’t going to get anything done anyway