Upgrading the Linux kernel opens a huge can of worms. Driver regressions happen all the time. The only things I think are worth backporting in an enterprise kernel are hardware enablement changes and security fixes. If a customer wants to use to a LTS kernel they can compile it by themselves. There is nothing stopping a customer that wants a new kernel from compiling it, and running it on RHEL. Red Hat has to pick their battles, I imagine a very small amount of users care about this for workstations or servers.