Was recently told that Assistant is finally coming to Nexus 5X/6P soon with next “major” update.
— Stephen Hall (@hallstephenj) February 2, 2017
Stephen Hall claims that Google Assistant will be coming to both the Nexus 5X and Nexus 6P with a major update. Whether this will be the 7.1.2 update or a bigger update like 7.2 is not known, but hopefully it will be soon.
However, if you’re not keen on waiting and are willing to get your hands dirty, you can get Google Assistant on any phone running Android 7.0 Nougat and up.