She has serious campaigning flaws. Character flaws are mostly hype. Just because someone hypes an item up for decades doesn't make it true.
Clinton isn't energizing, isn't dynamic, for some reason thinks she needs to shout every single word with vocal cords that can't take it any more, etc. These are not features that make for great sound bites or generate a lot of enthusiasm.
But, she is a known result. Most likely her reign would be fairly similar to Bill Clinton and Obama's reign. It would most likely be fairly slow, steady, stable. That alone has appeal compared to Trump.
When Sanders is gone from the picture, those who wildly supported him have a big decision to make. The known to be good for the US vs. rolling the dice. I suspect that maybe a quarter of Sanders supporters will just not vote, maybe up to 15% will go to Trump, but the rest will shrug and vote Clinton out of the fear of what Trump would do. To win, Trump needs at least 2/3 Sander's supporters and I just don't see any evidence of that happening.
Note: I am a fiscal-libertarian with a left-leaning social bent. Sanders would probably get my vote, but he just takes things too far for me to be a big Sanders supporter (especially his too-strong union support and anti-trade tirades). I'll probably reluctantly go Clinton.