The reason we won't get the OnePlus Z in the US isn't fun, but it is obvious: OnePlus has no reason to offer it when it has phones with much better margins in a rich country where people spend lots of money on phones. https://www.androidpolice.com/2020/06/23/oneplus-ceo-teases-upcoming-affordable-smartphone-product-line/

There's a pretty massive black hole in the US phone market from $300 to $600 these days, and while that market has grown, the growth has been slow. Carriers are largely uninterested, too, as payment plans make high-end phones "affordable" for many.

The only cheap phones carriers are interested in are the ones with extremely good margins for their prepaid brands. A $300 OnePlus phone is going to have terrible margins, no carrier would want it for prepaid, and it would sell zilch postpaid. That means open channel sales only.

And OnePlus is making it clear open channel is not the end game, but rather has been a way to get a foot in the door. Selling a phone in the US that would by default have no interest from carriers would do very little to help them in this market.