I think its a combination of, there being a reason why they are popular (sherb is adorable come on) and exposure to the villagers.
The more people talk about certain villagers the more aware of their existence people become.
Such as I know who Bob is because a youtuber has them as their favourite villager. There are lots of villagers who's names I don't know because I've never met them myself and no one goes on about them.
I mean I do agree with you. I think Bob is hideous and can only put the love for him down to a cult mentality XD
But yeah, I think its down mainly to what people know about and how some villagers just have wide appeal in general.
Bob became popular during New Leaf. He wasn't considered a great villager at the time. People were trying to get him to be loved by people in the AC community and obviously it worked. I don't find Sherb appealing personally because he looks like a sickly child to me. There are plenty of villagers in the game that I feel deserve more recognition compared to the current popular ones. But like you said, it's exposure.