One problem is that there is a basic cultural divide between the 2 countries. Now, I know there are exceptions, but American artists tend to sing about what they've got (just look at RnB, Rap, Hip Hop etc) while UK bands sing about what they haven't got.
The other main problem is size. America is so vast that it is impossible to be played on 1 radio station and be heard by most of the nation. In the UK, you can have your song played on Radio 1, and the chances are, a good percentage of the population will hear it. TotP would have done the same, if it hadn't been flushed down the BBC toilet. Cracking America takes time and money, as well as blood, sweat and tears, travelling from state to state (there are 50, remember!) and doing as much promo as possible. With a country so large and a population so high, even if you promote your butt off, you might only reach 1% of the population, and which record company is going to continue to throw money at such a money-wasting scheme?
Saturation is another issue. Why should Robbie Williams make it big there? I mean, he's a mediocre lounge singer who made if off the back of a dreadful boy-band. Um, who needs him when you have Justin Timberlake? The only UK acts who get any attention over there are ones who break the mould, like the Spice Girls. Even Queen were never a huge success in the states.