Out of all the possible scenarios, characters to develop, places to flesh out and ideas to explore, ZUN wants to dedicate another Manga to these three insignificant Danmaku fodder that add nothing to the lore as a whole? Sometimes I really wonder what goes through ZUN's head when he plans out his Touhouverse.
Most of the various lore related elaborations and explanations from the mangas, novels and CD stories are mainly from the title characters'point of view (to a certain extent) : rather than serving the lore related stuff dry, the books are more about exploring the Touhouverse lore through the characters.
Compared to the more juicy stuff that Bougetsuhou, CoLA, RenMerries stories, WaHH and FS gave, the daily lives of the three fairies might seem boring an unappealing ; but if you look at it in another way, it's a breather : a fresh and simple addition to the already heavy and rich material available. Even Bougetsuhou, which had a fairly serious tone, had a light hearted 4koma side stories. And I don't quite think that these 4koma had a significant relevance, lore-wise.
Besides, there are new characters and new settings that weren't there back in the time of the three fairies. There many way of having them interact with these new things : trying their hand at urban legends ? Getting to know Shinmyoumaru ? Interacting with Clownpiece ? Or just a regular random adventure ? It's a good occasion to tie the old generation 2hus with the new ones. Besides, the book is not out yet, so it may turn out to be anything. Obviously, it's gonna be a Slice of Life kind-of manga so there won't be anything over the top
It's not quite possible to say what ZUN's thoughts and objectives were when he chose to start this new manga, but as far as character choices go, I think that there's no need to introduce new characters that would fullfill the same roles that some preexisting characters already do : If he intended to make another Slice of Life manga, new characters were never needed. Granted, using more popular characters besides the three fairies might have been a valid move, but if ZUN chose these three then, hey, can't help it but trust the guy's judgment.
Anyway, it's not like we have a lot of information about this right now. It'd be better to just wait and see how it turns out, just in case this happens to end up being another Touhou-14.5-bashing-before-the-game-s-demo-actually-got-released.