Americans spell like handicaps, and to them, it's right.
No, doughnut is still widely used in America too. I believe the popularity of donut comes from Dunkin' Donuts. I don't know if they are in the UK so you might not know them; they are a large doughnut chain in America.
inb4fatAmericanjokeOn-topic: Do not want.
Maybe it's the faces, the crude marker drawing, or even the old meme.
Most likely it's the fact that the doughnut should already have filling in it and that it costs $15(75?).