I had nothing to do with the effort with getting it merged. But Junjiro Okajima (the author of aufs) seems to have tried a few times, and obviously it hasn't worked. At the same my observation as a "real world implementor" is that it works great and no obvious alternative is available.
Again I'm not an expert in "kernel politics" (you could even say I know nothing about them).
Union filesystems in general have had a storied and troubled past in the Linux kernel's political landscape. Valerie Aurora (formerly Valerie Henson), a former redhat kernel developer who worked on Union filesystems tried quite valiantly (no pun intended!) to get the unionfs patchset merged and ultimately left redhat to pursue personal things.
Somewhat recently there was some traction with a version of her patches that looked somewhat close to being merged, but it might have also been dropped.
