I think it's a personality issue in so far as Sanders is inflexible about his incorruptibility. In general his conflict with the brass stems from his conflict with the donors and monied interests, and his refusal to bend the knee to them.
Because he has an important message and policy ideas that candidates like Biden won't ever really address or explore beyond paying lip service at best.
And as to his impression on electability against Trump, the polling among the general population seemed to agree with him, even state by state at the time. As stated previously, and as has been proven repeatedly, primaries are not general elections, or a true test of electoral fitness for the general. A primary, more than anything else, determines who a particularly partisan and politically invested minority of a minority believes is best for the general; sometimes they succumb to their own collective confirmation biases and get it totally wrong.
Also I feel Bernie would be a far better candidate than an obviously borderline senile Biden, whether or not the Democratic party specifically agrees.
I was more speaking to the impact of independents and the importance of appeal to them personally, keep in mind that primaries are highly partisan environments with significant confirmation biases and bubble elements, but yes, people who sit out or defect per marriage to a candidate are also a substantial albeit less significant factor.