One reason for that, of course, is that you pay to receive an SMS message in the US, thus making this potentially a nice little earner for Google. To do a similar system in the UK would require either a premium SMS service, or to run it as a loss leader.
This is a dilemma a number of text based web services have come across - Twitter's UK SMS service for example limits you to recieving 250 texts per month to a UK mobile number, while it is unlimited in the US.
More at the Google SentTo We Page (via Pat Phelan).