Story elements are easier to pitch than gameplay, since gameplay consists of "everything in the game." I'm talking about not only the combat, but art assets, such as character art and the stages. All of this takes quite a bit of time to work on, and can change drastically over time as game design is an iterative process. While the story is also in a current state of flux, it is easier to share, since it doesn't necessarily require all of the aspects of game design to be at the same level of completion to be presentable.
Since the story is what we've shared the most, it's also what people have asked questions about the most during streams, since there's more existing groundwork for people to begin speculating.