When Franz Stephan von Lothringen married Empress Maria Teresa, marriage rules within the family were the well known AEIOU tu Felix Austria Nube. Then as they were having more and more children, many of the noble families of Europe and principally from the Austro Hungarian empire sent their sons and daughters to Vienna and tried by all means, to introduce themselves with the multiple and good looking Archdukes. They employed all types of tricks and the young archdukes and archduchesses were overwhelmed by the "attack" from all sides. All central European families wanted to become members or related to the Habsburgs.
In order to protect the archdukes from the constant attacks, principally by the "Petite Noblesse", and to continue with the idea of the archdukes and archduchesses marrying princesses and princess from other countries, the Habsburgs made rules whereby only 12 families from the Austro Hungarian Empire were "Ebenburtig", which means Ok for the Archdukes to marry. All other families were considered as "Not up to standard" or morganatic. This meant that if an archduke married a noble from the Austro Hungarian families that was not in the chosen list of 12 families the marriage was considered morganatic, which means that their descendants had no right to the throne and were no longer part of the House of Austria.
In order to perpetuate this defensive rule, it was written into the Austrian Constitution and never changed. This made that the Archdukes were no longer so interesting and the constant attacks stopped overnight. It was only the Emperor who could, under his authority, accept a marriage as non morganatic.