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.