Sometimes, an issue in the desktop environment needs some deep digging for proper fixing. Having to dig deep into each of the supported platforms is daunting and will probably lead to a worse user experience. So I think they should just pick their platform and work on it.
Nobody expects Apple, for example, to support the Mac OS X UI on different kernels/platforms, even though Darwin is very Unix-like. Apple actually go one further and only support their own hardware. Why can't GNOME focus like Apple does?