Seriously why did they not up it to 4.19? Do they hate bicycles? Do they make money based on the fact that upstream LTS kernels have short shelf lives compared to RHEL's own LTS kernels?
The kernel version was finalized some time before this release when 4.18 was current. Red Hat expends a ton of effort on long term maintenance and huge backports of new features to the kernel, so while I don't want to speak for the kernel team, I don't think the upstream stable kernels bring very much to the table.
Plus (my personal view) what goes into the upstream stable kernel is fairly random based on just mailing list NACKs, whereas what goes into the RH kernel has to pass a massive range of automated tests on a wide variety of real hardware.
Red Hat also certifies a whitelist of symbols that will remain identical in all future releases for the life cycle, stuff like that takes time so just grabbing the newest LTS kernel the moment they cut a release isn’t feasible.
Plus (my personal view) what goes into the upstream stable kernel is fairly random based on just mailing list NACKs, whereas what goes into the RH kernel has to pass a massive range of automated tests on a wide variety of real hardware.