The main issue is just that he gets overshadowed by all these characters that have fast, good strings, and safe options that are pretty consistent in all their variations. With Kotal, if you aren't going War God, then you are taking a HUGE risk by losing those versatile options he has in War God.
Just because he does a lot of damage does not mean he's gonna be viable in every situation, we actually have to work harder in order to get in and not get out. Kotal just needs a few adjustments in his moveset to complement that, because his other variations suffer for it. It's gonna make him weaker, predictable since everyone is gonna pretty much be forced to use War God in tournament, and sadly, less interesting.