They would have a really hard time selling a $35k+ car that performs significantly worse than a Mustang GT for one. The RS is kind of cool, but at the same time sort of idiotic. At direct exchange rates, it would actually tip the scales at $50k+, and even calculating the VAT and looking at the prices of roughly equal non-turbo Foci in US vs. UK to estiumate price, it would be at least $43k base.
Ford can get away with that with limited run vehicles in other countries where small cars aren't as looked down on, where FWD is even more typical, where cheap RWD performance cars aren't available as competition, and where the Ford brand is held in higher regard. Ford can't do it here. Even the ST is a stretch. They will probably sell 1 ST for every 100-150 normal Focus here in the states. It's a tough sell when the buyer looks over in the same dealership at a V6 Mustang that just obliterates the ST in performance for the same or less money. And for those that don't care about performance but want the hatch/compact look, they will get the titanium or SES instead.