From a researcher's perspective, upstream (Linux) kernel development is not attractive: it's a very time-consuming and lengthy process, often involving several iterations, which doesn't help with getting things done (i.e., PhD thesis and/or publications). Working around the kernel is much easier and, moreover, typically results in better performance, due to fewer context switches, better integration with the rest of the system, and more predictable behavior when running on other machines.

That said, some people in our group started looking into operating systems, but focus more on unikernels for cloud environments.

I mean, they built a whole kernel module that addresses page allocation/deallocation scalability issues. Whether that gets brought upstream, I'm not sure.