I don't know why George didn't tour the U. S. after his successful one of Japan with Eric Clapton even though he should have done it at the time.   I have seen videos of his live performances in Japan on YouTube doing Taxman & he looked healthy as well as playing great.  I haven't heard the live album from that tour yet except a few tracks that are excellent as well.  I think GH was afraid to tour America because of what happened back in 74 with critics & some fans.  His finest live performance as a solo artist to me will always be the Concert For Bangladesh but he missed a great opportunity especially after Cloud Nine & The Travelling Wilburys.  

I think that he wanted to return to the stage in a fairly low key manner, as he had done previously with Delaney and Bonnie. Given what the press did to him in 1974 I doubt that he would have wanted the publicty that would have surrounded a tour of the US, or even Europe.

He liked to have Clapton around him when he played live, and it's also possible that EC had other commitments, thereby preventing any other shows.

Just a theory.