I think it's one of those safe names; it has no real affiliation to any group of people (Redskins, Trojans) while still maintaining a sense of aggressiveness and intimidation. According to an unsubstantiated list though (http://highschoolnicknames.homestead.com/common.html)
, Wildcats is around fifth in terms of high school popularity, behind Eagles, Tigers, Panthers, and Bulldogs. I can vie for Panthers, as three schools in a neighboring conference of mine all held that moniker.
I tend to prefer the less common names. Michigan has some good ones, like the Jungaleers, Technicians, and Flying G's. Even my brother's is kind of unique (Dreadnaughts).