IIRC it was because VS2015 supported some of the ways you can use constexpr, but not all of them. I think perhaps constexpr functions worked, but not variables (?)
As far as I recall, VS2015 had some bugs with constexpr when it came out. So most use cases would work as advertised, but not all. I believe they have ironed them out now, so the C++11 use cases (constexpr variables and single-statement constexpr functions) should all work. After all, VS2015 successfully compiles Microsoft’s own STL which is full of constexpr stuff.
And obviously VS2017 goes beyond and adds C++14 stuff like allowing constexpr functions to have branches and loops, and declare and mutate local variables.