5) He is being used in the wrong position
Chelsea regularly deploy Willian as a right winger. I personally do not believe that is his best nor his favoured position on the pitch. If you look at his performances, you can see that he would offer much more if played through the middle in the number ten role, or even as a support striker who drops deep to collect the ball, before then turning and running at defenders.

I'm not claiming that Willian deserves to be a regular starter for Chelsea, as he has perhaps not earned that privilege back yet. However, I do urge supporters not to give up on him as I believe we are still yet to witness the very best from him.
He's reaching the peak years of his professional career and having been affected by personal problems lately, he's finally pushing through that dark period to assert his credentials as an asset for Antonio Conte.