I'm trying to understand how Linux Kernel development works and reading the relevant documentation at https://www.kernel.org/doc/html/latest/process/2.Process.html

So there is the so called next-tree accumulating all the patches from all the subsystems ready for merge and the mainline tree. The thing is the merge-window is opened for 2 weeks (as mentioned in the linked documentation) and as far as I understood all the changes will be merged into the mainline.

But looking through the next-tree I found some implementation in Rust https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/log/rust which is much older than 2 months. 

How is that happened that after a few merge windows (4 if I counted correctly) it's still not merged?