Cats are often regarded as sassy and alluring; they're sleek and mysterious; they're small, delicate, and cute. Catgirls are also, without the "cat" part, usually skinny, large-breasted, scantily clad women. They're sexy in a very trite, shallow way.
Basically, you could construct a female stereotype out of strategically picked cat traits (we conveniently forget "sleeping all the time" and "clawing your eyes out if you tick them off"), and the female body used in the catgirl image is already a Barbie-esque parody of a real girl. Hence why many guys fall all over them.