Hard to say as end customer - but if was supposed to make a guess it would boil down to:
Maybe @davidmoline can add some extra details ?
You kind of answer the question yourself. Although the architecture is similar it is not the same. There is cost and effort required to support different operating systems. This is the same reasoning as to why HP-UX and AIX have been dropped as supported operating systems for NetBackup servers.
I am not able to what will or may happen, but up until now I thiink there was not a sufficient footprint of the operating system to justify qualifying Oracle Linux. If this changes then the effort may be made. Making noises to your local account representatives can help to create the necessary push to make it happen.
Hope this helps.