I believe it's a health issue, or a being able to play guitar to a certain standard issue, not an audience issue.
Don't quote me on it, as it's only a hunch.
On musical exports, I think that has nothing to do with anything. The UK has always been 'cool' conscious, and a place where newness is more valued than legacy. Overall I think that's a good thing as it keeps the scene less stale.
For a very long time Mark has been out of step with music taste in the UK. A lot of successful UK artists are dance music influenced. Americana, folk, country etc have never been particularly popular in the UK.
In the beginning that worked for Dire Straits as when Sultans was released it sounded SO different to everything else on the radio.
It's the same with McCartney. He is still packing out huge stadiums in America, but hardly ever plays in the UK.