Actually, there are Win 10 mobile devices stuck in some old build.
Also, there are also some devices from some OEMs that are not supported in the insider program.
Re:Android security in 2016 is a mess
by ocdtrekkie on Monday November 28, 2016 @10:37PM
(#13060577)
Those are the cases of devices never supported with Windows 10 in the first place. Dev Preview still worked (for a while), but it never officially got 10.
Any OEM, for instance, that didn't give up control of software updates, couldn't get Windows 10. Even 8 had some more leeway, Dev Preview could install over the OEM build whether the OEM drivers were ready for it or not.