Board Thread:Manga/@comment-39898841-20190622181312/@comment-38964308-20200609145844

Because Ymir was forced to bear the first king of eldia's childrens,the power of titans is very connected to royal bloodline,that's why the full abilities of the founding WERE for royalty only,a double contact between a non-titan royalty (Historia) and Rod reiss (in human form) and a non-royalty intelligent titan (and holder of the founding) (Eren)  in the other hand unlocked some of the Attack abilities,in the same time,even contact between a non-titan royaty (Historia) and pure titan royallty (Rod Reiss) unlocked some of the latter's memories,that's because probably the kings of Eldia did some genetic engineering and experiments on Ymir's contrymens (Subjects of Ymir) and forced them to reproduce with a majority of his own people (majority of Eldians) to awaken their abilities to transform to titans,in the other hand, the other clans were immune because some were possibly granted to not be forced to reproduce with eldians in exchange of sending some of their people as experiment subjects (non-SOY Eldians),in the other hand,some clans were given responsibility to protect the royal family in exchange of granting them replica of titan powers without turning into ones (the Ackermans),this is my theory.