Because USA wouldn't issue a national security letter to get dirt on foreigners? Seems like if Mozilla wanted to choose DoH as way to protect people from being spied on they wouldn't choose USA-based companies?
Like "don't get caught by your own gov accessing restricted material get caught by USA gov-related organisations so they can sell you out to your own gov & benefit USA".
If you're bothered about who's watching your web traffic then it seems highly unlikely you want to add USA to the list of who is watching you??