Doctor Strange stars Tilda Swinton as the Ancient One, and while it upset some fans due to the character's comic origins, Marvel doesn't see it as a slight.
The company said (via Mashable): "Marvel has a very strong record of diversity in its casting of films and regularly departs from stereotypes and source material to bring its MCU to life. The Ancient One is a title that is not exclusively held by any one character, but rather a moniker passed down through time, and in this particular film the embodiment is Celtic. We are very proud to have the enormously talented Tilda Swinton portray this unique and complex character alongside our richly diverse cast."
Marvel doesn't see it as an issue, mostly due to the fact that the "Ancient One" title isn't one particular person, but an honor passed amongst many. While that makes sense, it doesn't explain why they chose to go with a Celtic take rather than a Tibetan one. Some of the issues are spillover from recent comments made by scriptwriter C Robert Cargill.
"[The Ancient One] originates from Tibet, so if you acknowledge that Tibet is a place and that he’s Tibetan, you risk alienating one billion people who think that that’s bullsh*t and risk the Chinese government going, ‘Hey, you know one of the biggest film-watching countries in the world? We’re not going to show your movie because you decided to get political," Cargill explained.
"If we decide to go the other way and cater to China in particular," he continued, " — if you think it's a good idea to cast a Chinese actress as a Tibetan character, you are out of your damn fool mind and have no idea what the f*ck you're talking about."
From Cargill's comments, it would seem going a different route casting wise led to an easier PR path for the studio in regards to its international audience. Marvel's statement merely framed it as a creative choice. Most likely it was a mix of both.