I apologize in advance if there's more than just a soupçon of snark- not trying to be hostile here.
Yeah I'm not seeing the whole has to be playable by your mum requirement.
There's a whole generation of kids out there whose parents (including the moms) raised them on D&D or other role-playing games, so they might be interested.
99.9% of readers will be wargamers, especially of how brew rules or niche publishers.
Space Weirdos and Song of Blades and Heroes, among others, would like a word. These games are designed for people with no wargaming experience, or perhaps coming from RPGs or board games.
There's plenty of room for both "games that I would only play at a convention with a GM" and "games that a new player can pick up in two turns". RPG and CCG players ask me all the time what I'm doing when I play Frostgrave at a store, and that's one of the reasons I play it- it's easy to teach, and it might mean a new hobbyist in the ranks.
It's not like GW releasing a new edition of WH40K that has to be playable by 12 year old kids so needs a gentle hand holding explanation.
First off, some of those kids are better gamers than you think. Teach them well and they'll be giving you a kicking before you know it.
Second, as I've said before, there's room for games of every stripe. I prefer a hand-holding explanation to the sort of ruleset that assumes you know how to do certain things because it's more of a cheat sheet for a group's homebrew rules that they know by heart (or it's just not written well). It shows me that the authors want me to learn and stay interested, even if I know how all of it works already.
Why fill a rulebook with un-needed fluff about miniature wargaming. Pretty much everyone will be coming to the game from a different more popular game.
Um, because good fluff is fun? It also explains why one might pick Faction D over Factions A, B, and C. Some players want a unique setting, like Alkemy, Malifaux, or The Drowned Earth. Others like a basic framework that lets them branch out, like the -Grave games. Some prefer a specific historical period because it's interesting to them.
Without the fluff or background, I might as well skip the miniatures, the terrain, and everything and just break out a hex-based game, an abstract board game, or chess. Have you ever talked to people who only like abstract Euro board games? They are smart, highly tactical players, but most of them are drier than yesterday's toast, or at least their taste in games is.
It isn't too much to ask for an author to give us a paragraph or two about why the rulebook exists.
I should also note that your want is being granted more often these days: more companies with heavy fluff are releasing it separately from just the rules. Multiple companies have the rules-only version and the complete version of their books. Take the lite book or PDF to the store, leave the big one with all the background at home to read for fun (or just don't bother with the big book).
The bottom line is this- if you can't teach a new player easily, or at least easily enough to whet their appetite for more, they won't bother. This is especially true in this age of endless distractions and instant gratification. We need simple games with hand-holding in order to bring in new blood; they won't stick around for the more complex games if it's daunting from the first step. At my local con, a lot of the heavy historical game players get older and greyer with each passing year, and seem to think that forcing their kids to help will keep the hobby alive (usually, they either look bored or they are amusing themselves by picking up the miniatures and playing with them). Meanwhile, the simpler, faster playing games have vibrant tournament scenes, and the easier-to-run participation games fill up quickly.