I will have to agree with many of the sentiments echoed here. Historical IF struggles in finding a market simply cause it’s a niche genre, for sure. But there is also a mass market appeal for it. Traditional war movies have always done pretty well, from WW2 to Ancient Rome and what have you. There are a ton of books out there as well. The challenge, I think, is in marrying and presenting all these different facets without inundating new players with jargon (something which hardcores tend to do), and playing somewhere along the middle.
While DnD, IF and many of the CoG/HGs tend to favour romance and fantasy (which sells amazingly well, why would you want to play as a boring realistic painful peasant when you could be an all powerful mage). I feel it is quite important to try and bring in some of these elements as well - ROs are a huge thing, power fantasies (leading an army/nation) or even action/battle scenes are huge sellers too.
warning - shameless plug
When I wrote Gladiator it was kind of on a whim / I didn’t see any gladiator related IFs around, but I think it has done moderately well for all its worth. A big part of it is giving the player a sense of control to deviate from history (freedom of choice - or the illusion of freedom) while retaining that authenticity that gives the game the flavour, and plants the player in a world that is somewhat historically familiar yet allowing for room to world build and explore. While I’m not an expert, the game (released 2? days ago) is now trending 3rd/4th on Role Playing games so that’s goes to show… Historical IFs are not dead!!!