I think he's (correctly) determined that making an appearance might do more harm than good. There's really nothing he can say at this point to redeem himself before that audience. And if he shows up and there's a scene -- even if it's only a handful of people heckling him -- that's negative video that will get a lot of attention. By comparison, the bad press he's getting for not showing up will likely blow over without leaving a lasting visual impression. So, declining to attend is probably the lesser of two evils.
The real question is why he let it get to this point.