Just taking a wild stab at it, but I'd assume the reason it hasn't been picked up yet is because there's already been so many "public domain" releases of this film in the US? Perhaps they're worried that their's won't sell?
EDIT - Yeah, kidnicky already said that

But to answer his question, most
average Godzilla fans probably wouldn't buy the same movie twice on DVD. The die-hards, yes, but then how many are out there? Not saying Sony are right in their assumption, but that might be the way they're looking at it.