Granted, US English simplifies some of those more insane pronunciations with the bulldozer approach (of either pronouncing it like it's spelled, or spelling it like it's pronounced) especially as you go further west.
And, "donut" could easily be parsed and pronounced as "do nut" by someone unfamiliar with the term (granted, it'll be a child or a non-native speaker that'd do that), whereas "doughnut" will in US English always be parsed and pronounced as "dough nut".