Xiaomi has never officially sold any smartphones in the US — one reason is because the phones aren't built with the US' 4G LTE networks in mind, as Xiaomi VP Hugo Barra said during Mobile World Congress last year. Xiaomi would have to redesign some of its internal components to support the US market.
But there's more to it than that. Xiaomi is still a small Chinese company — it's not even the top phone maker in China anymore. So, without conquering its home market, selling phones in the US would be an uphill battle, especially since Xiaomi doesn't have any relationships with any of the US carriers, and would have a tough time selling its cheap smartphones in a market that's largely been saturated by Apple and Samsung devices. Xiaomi will likely keep its focus in China and the general Asia region, including India. There's a chance Xiaomi could attempt to sell phones in the US at some point in the future, but right now, the company will only sell accessories and smaller-scale electronics, like TV set-top devices, here in the states.
Still, it would be great to see the Mi Mix in the US at some point.