A slow maturing story based theme would be ideal, but from a producer point of view it is costly to start. I can’t speak for Lego, but I would imagine that it is a big risk to heavily invest into theme, getting writers to create a story, promotional TV series, online games, ads. And even if they do all that, a series might fall short, like Nexo Knights.
About the successful story plot Lego creates, I once pointed out before that: