Elizabeth is untouchable in MA, they want her as their Senator and that's why she came in third in MA presidential primary, they want her to represent them forveva eva, foreva eva.
It's not that (Warren's approvals in Massachusetts are consistently anemic for an incumbent Democratic Senator, although since it's Massachusetts that means they're still pretty good), it's that there really is not any room to Warren's
left (which is where a Bernie-backed primary challenge would presumably come from) in statewide Democratic politics here.