Because it's not more accurate, but less. :)
Just as 'messiah' comes from the Hebrew for anointed and doesn't mean divine; Christ comes from the Greek word 'Christos', also means anointed and doesn't mean divine.
David for example was anointed; if we spoke greek we could say his mother was the Mother of Christ. And, I've heard many a minister claim his anointing by God. If they were Greek speakers they would be saying Christos.
So the "Mother of Christ" doesn't say fully what orthodox Christians believe Jesus really is.
So I'm learning. It's been a real eye opening experience.