The Immaculate Conception is often explained by a container metaphor, i.e. holy things are kept in holy places, fine wines are kept in perfectly clean vessels, etc. The metaphor has value but has all the usual limits of metaphor and fails to address the ultimate reason for the doctrine.
The limits of the metaphor are that (a) being a mother involves much more than being a container, and so to explain Mary qua container fails to account for her actual role as mother of God and (b) any argument for Christ not entering into a sinful womb (considered as a container) is an argument against entering into a sinful world and so is an argument against the Incarnation. Obviously, any defense of the Immaculate Conception that argues against the Incarnation has made a wrong turn somewhere.
But a mother involves a more than containment: it involves being the source of existence for a person. In Mary’s case this involves being the source of existence for a divine person, which requires a ne plus ultra degree of elevation by grace. Given grace is incompatible with original sin, Mary’s maternity is incompatible with any degree of it also.
To flesh out the main idea: grace is not simply a restoration of human nature but an elevation to a properly supernatural and divine life. Sanctifying grace can just as easily be called deifying grace. Mary’s act in salvation history required a degree of grace that made her in a robust sense equal to the Father, i.e. a degree than which nothing greater can be thought. Given grace is a remedy for the loss of original justice Mary escapes original sin to a degree than which nothing greater can be thought.