All Sanders did was show he's the only candidate left not dodging a debate. Both Trump and Hillary running away from him now. He'll still lose though.
Hillary and Trump both won or tied all (or all but the CNN one with Carly in Trump's case) of the debates they were in. Bernie didn't win any debates. I'm sure they're not afraid of him. The problem is that he can hurt either of them in the real game whereas a victory over him only helps them in the imaginary game against him. It'd be like if the Panthers took a break from the superbowl to go play the Cowboys, but every point the Cowboys scored went to the Broncos. Sure, they'll win 42-7 or something, but that 42 doesn't count for anything because the game with the Cowboys doesn't count, while the 7 points to the Broncos could matter a whole hell of a lot. It's not cowardice to turn down such a silly deal, and it definitely wouldn't be because the Panthers were afraid of the Cowboys.
Good analogy but that's why I knew Trump's people would talk him out of doing this. (Also, Bernie would do much better against Trump than Cowboys against Panthers.)
The last thing Trump needed was to see the diehard Sanders supporters, now riled up against Hillary, to tune in en masse and get riled up by Bernie against him and help unify Democrats. Bernie will endorse Hillary in a few weeks or at the convention and maybe will eventually do a bit of what Warren is doing now. But it'd be a whole other mess for Trump to have a massive national audience, with many Bernie Bros, watch Bernie fight with him and make him look like a phony. For the same reason, I don't think Hillary was ever all that worried about this hurting her if Trump was crazy enough to go through with it.