In most cases, libraries are only included in distributions such as RHEL because they are required by another package which is desirable to have in the distribution. This is always worth bearing in mind as a general rule: the packages available in a distribution, in particular libraries, are there for the distributions’ purposes, which is to ultimately provide a working environment for end-users, not as general-purpose development tools.
So the basic answer is that YACS isn’t included in RHEL because nothing else in RHEL requires it, and no end-user use-case was made to support its inclusion.
A consequence of the general rule I mentioned above is that libraries included in a distribution aren’t necessarily appropriate for third-party programs. In Python’s case in particular, it’s often worth using virtual environments instead; those can have whichever Python modules are required, without affecting the system’s.
(I’m ignoring Software Collections here; these are designed for development use-cases, but they still don’t include YACS.)