I love when people think pickrates only relate to the champion's viability...
Without even considering the fact certain champions aren't popular due to thematic reasons. Zilean, being an old man, doesn't exactly have a large ideal audience. You could make arguments towards the doctor from Back to the Future and stuff, but for most people it wouldn't really sell him.
SImilarly, Urgot on his own wouldn't really be that popular. In my experience, most Urgot players picked him up for, memetically, being one of the worst champs in the game. Some found his kit actually cool, others just troll with him. Very few thought about buying him when first skimming the characters.
Meanwhile, even when some champions are low tier, they still have decent pickrates. Olaf and Eve come to mind, who held fairly high pickrates even when they were borderline unplayable.