I think itīs mostly because of the close family relationship. For example was the second wife of emperor Franz II./ I., Maria Theresia of Neaples, Franzīs cousin, a daughter of Maria Karoline, daughter of empress Maria Theresia, whereas Franz was the son of Maria Theresiaīs son emperor Leopold II. (only one example). But it is strange that there are always examples, where single members of the family do not have this typical "Habsburg face" (or at least not so strongly, I remember especially the daughters of Philipp II. by Elisabeth Valois, but there are also other cases) but in general that looking passed so many generations. But in my view Franz Karlīs looking was not that longish like that of his brother Ferdinand, he was even much worse in looking. When Ferdinand was young there are quite nice paitings, but when I once saw a late photography of him I was really shocked
. When you look at the children of Franz Karl and Sophie is obvious that all look somewhere different, but Ludwig Victor is mostly "Hapsburg" of all (and Maximilian of Mexico also a bit), but Franz Josephīs features are better, they may also come from his Wittelsbach side?! Even when I donīt see any direct resemblence to his mother, Archduchess Sophie.