The Unicode Consortium Discussion Forum

Why the CJK Compatibility Ideographs are not compatibility decomposable characters ?

This is addressed in section 2.3 of the Unicode standard. There are basically two factors here. One is historical. The other is that variation in Han is a lot more complex than it is in most other scripts. Another, preferred, mechanism exists for dealing with Han variants, namely variation sequences. The Unicode consortium is encouraging the various members of the IRG to use variation sequences instead of compatibility ideographs in their future work.