I know the easy answers, I want the hard ones.
I believe it is frequently omitted because it is the most explicit expression in the Mass (along with Canon I) of the
cultus of the saints:
Ideo precor beatam Mariam semper virginem . . . as well as being a frank and unambiguous acknowledgment that we are all sinners who need God's grace.
Most liberal priests find it intolerably old fashioned and omit it because it contradicts their theological prejudices.