A journal diary starting with the birth date sounds like a great way for someone to get into journaling, those who make resolutions on their birthday and want to get into journaling or already have the habit could be very interested in it.

One way to find that out would be through the folks at r/Journaling.

Business wise, I presume It would mean 366 different diaries to be made for each type of diary; No changes in printing but in making the diary(arranging the pages) and  maintaining a large inventory.