"If she really wanted her children to marry fellow-nobles she would have given them another upbringing, like the Belgian-based catholic royals/nobility. Instead her children seem to have become creative and people of the world. Not hiding in their own cocoon with their fellow-nobles/opus dei members".
I think Marengo is right
. Although Furst Johannes was quite social, the people he mixed with were quite "high society" types like Princess Soraya and Gunther Sachs. Gloria's aquaintances have always been a more diverse bunch.
I think there was an effort by Gloria to make MT, Elisabeth and Albert "citizens of the world" and to mix with people because they were interesting, and not because they came from the same sphere as you. The children when they were young seemed to visit the US a lot with their parents and apparently were looked after by a Scottish nanny for most of their childhood.
To broaden their horizions Gloria sent MT and Elisabeth to a British boarding school for 2-3 years, whilst taking Albert away from his school Regensburg to complete his schooling in Rome (although I think he almost went to school in Britain too). Later on ofcourse the girls were educated in Spain. whilst Albert went to university in Edinburgh.
Considering that the US and Britain has such a pull for the family, it makes sense that both girls might settle down with an American/British husband in a major capital city rather than with a Graf on a small estate. I'm sure whomsoever they chose will be perfect.