Perhaps, non-reigning houses never bothered to change their old marriage laws. In order for the heir to inherit the titles, head of house-ship, and fortune, they have to old marriage laws, so they have to search for royal or noble brides. For example, Hereditary Prince Gustav of Sayn-Wittgenstein-Berleburg, son of Princess Benedikte of Denmark, cannot inherit his titles if he marries Carina Axelsson because of the stipulations of his paternal grandfather's will. Prince Pedro Luis of Orleans-Braganza was said to be looking for a potential royal/noble wife for the future before his untimely death last month in accordance with dynastic law. Now it looks like his younger brother Rafael will have to do that eventually.
On the other hand, reigning dynasties, in order to be more revelant in modern society and closer to the people, have allowed for marriages to commoners. Also, many of these reigning families, their marriages and succession rules are based on constitutional approval (e.g. the Netherlands) rather than dynastic law.