I'm not a supporter, but I can see the reason.
1. She is what the democrat establishment wants as a nominee. She seems inevitable, so why not toss your support behind the supposed winner?
I do not think she would be that big of a deal if there were another viable "mainstream" candidate in the ring on the D side. Sanders may appear viable now, but he might not have that appeal in a general election. I do not think Sanders appeals to the bulk of rank and file moderate democrats. It's the radical left wing carrying him at this point.
2. She has a "D" next to her name. No matter how bad the candidate, party loyalists would rather hold their nose than to pick a candidate (even a possibly better one) in the opposite party. Not that I am claiming it exists here, but it can.
3. She's untouchable. It's pretty much assumed she'll walk away with at most a fine from this, if not a full, possibly preemptive pardon. it does not matter if she looks dirty, it comes with being a Clinton. It's expected of them.
4. Nothing will pull the democrats together more than the prospect of losing the White House for the next 4-8 years with all those aging Supreme Court justices waiting to keel over.
It's not just the Democrats going through this now. The Republican Party wants its chosen candidates and they get Trump and Cruz instead. Both parties are having to fight popular sentiment to get their candidates in the general election.
Watching both parties try to thwart popular opinion is proving to be quite entertaining, but not nearly as entertaining as a Trump vs Sanders in the general election will be.