Why is it that Zilean is seen as unviable? I know that he can be used in certain situations, but why is he so underplayed?

He's not unviable. Alex ich plays him mid competitively.

He can be played really well. Chat unhappy times in game. He'll help you.

Then why is it that very few people use him?

I main him as a support.

He's not unviable, he's clunky and cumbersome like orianna and anvia but he doesn't put out like they do cuz he's not designed to do things that stand out that way.