What makes a champion viable??

I was thinking about this today and i was wondering, why are some champions so played over others?? for instance lets look at 2 adc's

quinn and caitlyn

quinn has a blind and an escape, her ult is agreeably less than helpful in a team fight if you're playing as a traditional adc

Caitlyn on the other hand has a escape and a snare. but her ultimate is equally useless in a teamfight (you lose alot of damage stopping to snipe someone)

so my question is, why is cait picked so much over quinn.. is it that players dont build quinn in a way that shes strong right now or they dont use her correctly, or is a actual balance issue?? there are many more champions i could use as an example but these just come to mind first. specifically because i really enjoy quinn but i get metaphorically crucified even when i pick her against someone she isnt "countered" by.

dont get me wrong i recognize different kits different strengths but am i missing something or??

sorry about punctuation im on a phone and regrettably lazy