Why doesn't RedHat, or Oracle, or SUSE, or someone else run Linux through the compliance tests?
Primarily? Because it won't pass the testing without a lot of work. In particular, there are negative assertion tests on header files (some things are not allowed to be dragged into the namespace, and the header are promiscuous). There's also a whole bunch of testing having to do with full and almost-full devices. There are also signal issues and process group membership issues. For example, you can "escape" an exclusion group on Linux by setting your default group to one of your other groups; Linux overwrites the membership in cr_groups as a synonym for cr_gid, and doesn't handle POSIX saved IDs quite right, either (Neither do the BSDs, so this isn't a Linux-only problem).
Last time I attempted to run the test suit on Linux as a lark, there were about 20K failures (mostly tests not compiling because of it bailing out over the header file issues. There are also some parts of the system that have been subsumed by systemd; this isn't intrinsically a problem on its own, so long as the system *also* supports flat config files as an addendum, at least for some aspects of logging.
Also, getting the UUCP to work over USB serial dongles is likely to be something of a bear, unless you make the HDB modifications for handling the "rung indicate" as a notification to take the shared file lock on the callout device so the getty's don't start trying to chat with each other.
Finally, there some considerable legal/licensing issues for the trademark.