I can think of a few reasons why that would be the case with NanoFate. The thing is, while the anime never explicitly describes them as lovers, it does I believe have them in an amazingly close rock-solid relationship, cohabiting, and raising a daughter together. So there isn't a ton of room for plot there--the doujin world basically considers them wife and wife because, well, they pretty much are.
It's not quite like that, most NanoFate doujins have more plot than this one, but like in any other fandom there are doujins or artists that are more about showing an erotic moment of a couple than telling an story, and that's fine too, to each their own. Also after thousands of NanoFate doujins we have only so many stories that haven't been told, and as you pointed out there isn't much conflict to write something too innovative anymore in the happy married life of Nanoha and Fate, because that's what they're in canon now: wife and wife with a daughter that may follow their steps someday.
last edited at Jan 8, 2018 10:43PM