Tsipras has promised not to enter coalition with others than KKE. If he breaks that promise he risks defections from his left wing (which will be bigger than any potential coalition partner), so it wont be worth it.
If KKE says no, and the only way to obtain a majority is through To Potami or ANEL, he will break the promise. I'm sure a SYRIZA minority government would be fine for a few months, but what happens once they have to push their first controversial vote through parliament, or if a few MPs desert? If Tsipras wants to survive as PM, and actually accomplish something of value, he needs a coalition partner, and KKE isn't it. It is just a simple reality, and an election promise can't change that.
I believe that many of KKE voters are ones of who want KKE not to be irrational opponent of Tsipras, but a left-wing check on his policies.
