Maybe One plus has seen what happened last time a big Chinese phone maker tried to lunch a phone on the US market, with a US carrier. Thy got shoot down the day of the AT&T lunch, then their CFO got imprisoned in Canada on charges of using a bad PowerPoint presentation in a US bank, then the US government put them on a black list so Google couldn't work with them, and now they have been band from using US designed chips from Taiwan. So maybe One Plus thinks that they better not draw US government attention to themselves.
But I might be way off. ;-)