Lunarpunk is a relatively new and still underdeveloped subgenre born, or rather in the process of being born, out of solarpunk as well as science fiction and fantasy more broadly. A good way to start conceptualizing lunarpunk at a basic level is to consider it in terms of a basic and analogous relationship with solarpunk, particularly when it comes to the aesthetic aspects of the two subgenres.
I think “luna” sounds a bit more dreamy, which is what I associate with the lunarpunk vibe, maybe that’s why. But compare with solarpunk (and not solapunk or something), it makes sense!