I think that besides the conventional JW rhetoric - (idols, adoption of pagan holidays, trinity, and what-not) - there is a hidden psychological reason:
I think that the JWs are desperately jealous of the Catholics. At least the JW leadership - after all, Catholics are rich, well established world wide, and have probably the most valid claim of any so-called christian religion of actual historical linkages back to the early days of the church.
Parts of the JW system of rule are practically like Catholics...they have a sort of priesthood (elders), a conference of cardinals (governing body), and arguably in times past at least, a pope: as with Knorr, Freddy, Rutherford, or Russell. JWs like massive church property as much as do the catholics. JWs have created a sort of celibate monastery life at their bethel much like a Catholic monkhood. JWs pretty much have gone along with the bible canon selected by the catholics in centuries past. They excuse their dislike for other catholic doctrine as a falling away on the part of the RCC (as per the debate between Arianism and Athanasius).
It is true, though, that most JWs think that the Roman Catholic Church is the primary harlot of false "christian" religion, all others being not quite so bad and only secondary whores for the false lord.