The Pixel and the Pixel XL are the two newest smartphones from Google. In fact, in many ways they are a whole new type of phone from Google. While these are taking the place of the Nexus devices this year (and probably in the future as well), these are not Nexus devices. They are smartphones which not only adopt the Pixel branding, but also make use of a number of fundamental differences when compared to the Nexus line.
Most notably of which is the software and specifically the inclusion of the Pixel Launcher. While Nexus smartphones come running 'stock' Android, the Pixel phones come running a slightly tweaked flavor of stock Android, in the form of the Pixel Launcher. One which includes a number of notable changes like a Google-branded 'pill' icon, the lack of an 'apps' icon (you pull up from the bottom of the screen to access the apps) and more. As the name of this tweaked version of Android suggests, this is a whole user experience that has been somewhat purpose-designed for the Pixel and the Pixel XL.
And it looks like that is exactly how it will stay. It has been questioned as to whether Google would make the Pixel Launcher available to other phones in the future, much like the Google Now Launcher currently is. However in comments picked up by Android Police, it seems that is unlikely to be the case. To be clear, Google is reported to have firstly stated, that for now this is a Pixel phone-only launcher. However, they are also reported to have started that they are 'evaluating' whether to make the launcher available to other devices. So while it is possible that Google would open up the Pixel Launcher at some point in the future, the fact that they are only evaluating it at the moment, does highly suggest that they have no firm intentions or plans to make the Pixel Launcher widely available at this point in time. At the moment, this would also seem to suggest that is largely the case for Nexus devices as well, as no distinction between Nexus and 'other phones' were noted.