Well, as I alluded to in a previous post, almost by definition the "establishment" (which generally connotes the center of a particular organization) is not going to be in agreement with the extreme elements of that organization. Sanders embodies the left extreme of the Democratic party. I don't consider "extreme" to be a bad thing, just a geographic region of a spectrum. The median and the edge aren't going to be in agreement--on policy, but that bleeds out into the hazy concept of "electability." Because the mainstream Democratic party disagrees with Sanders on policy, they also feel that his policies won't be as popular.
A lot of Democrats do like Sanders' policies, but even they are split on whether to support him because some of them aren't sure the rest of America (which has been characterized as a center-right nation) will embrace his policies and they want to win the election. You can't do much if you don't win.
Similarly, a lot of Democratic House and Senate candidates who are running in "red states" may privately like Sanders' proposals (or they may not) but they certainly can't endorse them or him and hope to win in a conservative state. They also worry about running in a conservative state with Sanders at the top of the ticket, because it could draw out of a lot of angry/concerned conservatives to vote down Sanders and that could impact down-ballot Democrats.
I don't necessarily agree with all these concerns, but those are roughly the reasons why his ideas could be popular or interesting to many Democrats and yet most of the Democratic establishment is against him. And most of the Democratic voters, for that matter.
