Lack of Form - Illicit versus Invalid

I am looking for theological underpinning of why marriage of Catholics outside of the church is invalid rather than simply illicit. In reading Tametsi from the council of Trent, the primary purpose was to keep clandestine marriages from taking place and hence requiring them to be performed as a public act. It also prescribes the publishing of banns of marriage which are no longer required so this seems more a mater of discipline rather than doctrine.

I guess I am trying to understand the theological basis of why a marriage between two protestants could be valid if they are married by a justice of the peace, while those same two people if they are Catholic would be in an invalid marriage. This seems to be more a matter of canon law rather than a sacramental impedance so why is it not simple illicit for Catholics to marry outside the canonical form?

I am familiar with canon law in regard to this as well as having read Tamesi and Ne Temere, but none of these explain the sacramental theology on why canonical form has only been required for the last 450 years. If it wasn't an impediment for the first 1500 years, why is it now?

Just looking to understand how canonical form impacts the confection of the sacrament of matrimony. Any one that can point me to a discussion on the sacramental theology behind this would be great.