It's the way the kernel-development process works: As soon as a release takes place, there's a period (the "merge window") where new things can be merged in for the next release.

See https://www.kernel.org/doc/html/latest/process/2.Process.htm... for more details.

To make merges go smoother, there's a set of trees that are designed for developers to stage things that are ready to merge during the next merge window. linux-next is a good example: https://www.kernel.org/doc/man-pages/linux-next.html The Linux Networking folks have their own trees that fulfill the same role for their stuff: https://www.kernel.org/doc/html/next/process/maintainer-netd...