Boring old economics
In purely financial terms it is better to spend money at home. If the UK gives a US contractor a job the money is gone. Give the same contract to a home supplier and you collect tax from the company, the workers, and all the people who support them. I have been told that the difference is about 30%, so unless the foreigner is willing to work at those sorts of discount it is actually less costly in the long run to source at home.