I think part of it is because a lot of these more user-oriented features (desktop compositing, sandboxed app stores, hdr, etc.) don't get a lot of attention on

I think part of it is because a lot of these more user-oriented features (desktop compositing, sandboxed app stores, hdr, etc.) don't get a lot of attention on Linux until after the two major commercial operating systems get them and prove people actually want them. The monied interests driving a lot of Linux development do not have the same need for these things as end users. To them, Linux is a server, or a lightweight embedded OS, or needed for some other specialized use case.