Is there any reason why Vel'Koz isn't played except for lack of mobility?

People still actually think he's bad; it's unreal.

Because overloaded ass Viktor exists.

There are champions that have absolutely no reason to not being meta except for a really old excuse people have always used against them. Introducing:

skillshots

lack of mobility 100%. another factor could be how his AOE dmg is very linear . Maybe circular AOEs, like Brand are better than Line AOEs.

There are a lot of champs that out damage him

I feel like people just don't like poke/siege mages in general. People want kill pressure or massive combo potential and Vel'Koz doesn't offer either of those reliably enough so people just feel like they're spamming to no real avail and it's repetitive/dull. I don't think people care that he's immobile.

Because, nothing is less fun, than playing an immobile mage. Fighting a mobile assassin.

Then feeling completely cheated, because they can simply avoid your abilities with their mobility and kill you with basically targeted skills.

While Vel can be pretty OP, he's simply not a champion you just pick up on whim. Also preferred playstyles exist.