My guess is their would have been mistress jokes over the line if he attended, can't risk it.
I bet the have some internals and it wouldn't surprise me if he is toxic to their efforts and he will be riding the pine as they say in baseball until 11/8 just like Moochelle did because she didn't help the cause...
Interesting take. They probably thought Trump wouldn't have been able to contain himself and would have extemporaneously let a few comments fly about Bill. . and maybe Hillary.