I did a little research and found an article in PLAYBILL from a few years back about the various Normas in major productions of the show, focusing mostly on LuPone, Close, Buckley, and Paige. In that article, it was written that Andrew Lloyd Webber was unsure that Paige could be effective in the role of an American. If that is true and that's what kept Paige from playing the role earlier than she did, it makes no sense at all. Paige had already had great success in two ALW shows playing an Argentinian and...well...a cat. And she'd also had a great success just a few years prior to SUNSET BOULEVARD playing Reno Sweeney, a much more aggressively American character than Norma, in the original London production of the Lincoln Center ANYTHING GOES revision.
But, if it's true, it does go a long way to explaining why the first three Normas were American.
In fact, I don't think there's anything in the text of SUNSET BOULEVARD, the movie or the musical, that specifically identifies Norma as an American. There's no reason at all that she couldn't be an expatriate Brit in Hollywood. In any case, I don't recall Paige having any issue playing the role as an American.