The Mirrorball Man wrote:
I don't think it has anything to do with people "being at different stages", because that would imply that every actor's dream, every actor's motivation is to "make it big", to become popular and to star in summer blockbusters.
I think part of it is the stereotype of actors wanting to move to the United States to make big money. Arguably, the United Kingdom is the source of the richest, most varied and most original English-language movies and TV shows in the world, so a desire to leave that and go to the U.S. is seen as giving up the craft for money (whether such as assumption is actually true or not). Both Matt Smith and Karen Gillan are moving from the U.K. to the U.S., so everyone assumes "it's for the money". Hence the comparisons.
I suppose it's perfectly possible that the U.S. has similarly high-quality programmes, but if so, they don't advertise them nearly as well as the U.K. To a non-American non-British person like myself, U.S. programming means sitcoms and exciting "popcorn flick" movies (not that there is anything wrong with that---I enjoy many American programmes), while U.K. programming is more cerebral or interesting (Inspector Morse, Doctor Who, Yes Minister, Red Dwarf, etc.)