Not really sure why I've not been successful in the States. I don't think I can put it down to my research interests as I only applied to schools that had good research matches to my own. I have wondered if US schools prefer students from US schools both because they can easily judge the prestige of the schools as well as identify the referees easily. Whilst my referees are pretty well-known here, perhaps they're not so well known in the states.
On my own profile, maybe my GRE scores aren't competitive enough. I got summa cum laude for my undergraduate and have 4.0 (so far) for my MA so I doubt that was a negative factor.
Again, all just speculating though.