Four big things hurt Franken. First, the photo made the allegation seem believable, even if it had little real bearing on the evidence. Second, Leann Tweeden came across as sincere and didn't seem particularly opportunistic or overly partisan, even though she's a conservative. In fact, if I recall correctly she didn't even call for his resignation. Third, #metoo was at a fever pitch at the time, and Democrats felt they had to take a purist and militant approach. Finally, there was little reason to stand by Franken. His state had a Democratic governor, so there was no political price to pay by tossing him. Furthermore, though he was well-liked by senators, he actually wasn't a big media darling, wasn't particularly powerful, and wasn't really in anyone's back pocket, so there was no natural set of potential defenders. In short, he simply didn't have a lot of leverage on the key leaders from either party.