The reason is beside the obvious facts (like he is slow as a striker for EPL) he is not good at hold-up play, link-up play etc. at Everton, those were what he used to do perfectly at Besiktas. What makes him special is his finishing ability so far but that alone is not enough.
I mean DCL is a promising striker and maybe will be better than Cenk in the future but I don't think he is currently and as far as I see he is the 2nd choice striker. Well, if Cenk dropped to 3rd choice, it is not good for him. Plus, because of the current situation I think his morale is pretty low and he looks like he has lost his self-confidence already.
Can he still be a good asset for the team? I am sure he can but eventually being a 3rd choice problem will drive him to the point where he needs to make a decision.