I mean, on the one hand, Beto is legitimately the least progressive candidate in the race. Leftists in Bernie's camp have a legitimate interest in pushing back against that if they want to push things a far to the left as possible.
On the other hand, I think this Nate Silver tweet points at something real:
Bernie was only able to get as close as he did on the backs of a combined effort from committed progressive voters, and younger, more disengaged voters who care more about the general sense of energy and personal appeal of a campaign. Beto is a unique threat since he seems positioned to take that second group from Bernie in a way the other establishment candidates can't. Beto seems positioned to serve as a pretty fatal dagger to Bernie in the primary, even if he can't actually win for himself, so there's even more incentive for Bernie supporters to lay down the case that he can't win now and to keep him from running altogether.