Maybe it has to do with who the people involved think is best-suited for a role? Katie Finneran was hardly a star (at least, by this thread's limited definition) when she opened
Fuddy Meers in the West End, but Sam Mendes (whose company was producing) was adamant about wanting her. I've heard that Redgrave and Jones were pretty vocal about wanting Gaines to come over to London with the production. Conversely, I think a lot of the buzz around the recent
Sunday revival had to do with the performances of the two leads, which is why they were brought over but the entire supporting cast was American.
As others have pointed out, this thread's definition of "star" is questionable. Tyne Daly is a star--because she's familiar from television, I assume--but Boyd Gaines is not? I'd say Boyd Gaines is definitely a star in the world of theatre, which is precisely what we're talking about here.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body