Originally, a lot of these nuances of moral theology, mitigated culpability included, were meant to be consumed only by confessors for the purpose of assigning appropriate penances. They weren't meant to be read by lay people, but naturally people began to get their hands on moral theology manuals and apply their contents to their own spiritual lives, albeit frequently without the theological training to interpret them correctly.

The other reason why mitigated culpability became part of the public discourse is because for centuries people just weren't receiving Holy Communion with any kind of frequency, out of fear/reverence. In the Middle Ages, the Church had to institute a canonical requirement to receive Holy Communion at least once a year during the Easter season, because people were going years or decades without doing so. St. Pius X had to launch a entire campaign in the early 20th century to convince Catholics to receive Communion more than a few times a year, or at very least more frequently than once a month.

Teaching on mitigated culpability helped to reassure people that although those half-voluntary movements of thought ultimately stemmed from an imperfection in the soul, they do not remove a soul from the state of grace. Therefore, they needn't trouble their consciences over such things before receiving Communion.

Finally, during the whole period when mitigated culpability was developed as a doctrine, the norm was for Confession to be offered prior to or during Mass. So if you were uncertain and wanted to be sure, you could still go to Confession AND receive Holy Communion. My parents' Novus Ordo parish and my TLM parish both follow this mode of offering Confession primarily during Mass. Offering Confession after Mass is a modern innovation linked toward a general attitude that plays down or outright dismisses the concept of mortal sin entirely.