Not to overly generalize, but dog girls just look goofy. Cat girls look sexy. Cats are usually seen as elegant, powerful and mysterious. They are nature's natural born killers. But graceful enough so that fashion models do a "cat walk". Dogs are usually wild, spastic and needy and will lick you with unending excitement.
And cats are probably more popular in Japan because they are very clean and low maintenance. Average Japanese are overworked and don't have the time to walk a dog multiple times a day.
As for higher prevalence as a sexual kink? I'd say the more socially isolated/repressed people become, the more extreme the kinks. Like the Germans. And cats are a natural progression of that. Anyway, that's my guess.