How come islanders don't have knowledge to create titan serum even though Reiss family have them?
This might be the knowledge that only the Fritz family knew about, which Grisha Jager killed in the first place. or maybe, only the wielder of the founding titan from a royal bloodline knows how to make it. As you might know, every royal who inherits the founding titan also inherits the previous wielder's memories and king Fritz's will of peace.