I'm a bit out of the loop on kdbus, why didn't it get merged? It was my understanding that it had a brief stint in linux-next, so it must have been getting close...
It was so big not many people wanted to review it properly. There weren't many steps of implementation (essentially a single-patch dump) and it had some high level features which don't necessarily belong to the kernel. Developers of kdbus also brought up performance gains a few times, even though it's been pointed out the userland is not the problem. It was the actual implementation that was bad.
It's being reworked into bus1 which is a slim, generic bus implementation. Full dbus can be implemented over it, but it's not forced on people who just want a nice, light bus interface with some security guarantees.
No, because it's an overcomplicated system that's putting things into the kernel that definitely don't belong there, because the userspace implementation was slow, which, according to Linus, was not an inherent problem with the idea of putting it in userspace, but was because the code was written by a bunch of monkeys at typewriters.
[0] http://www.bus1.org/bus1.html