The initial reasoning from the OP sounds like it's running from the assumption that game rules and systems are the selling points, when I think in reality it's IP and story potential that catches people's eyes and gets them to investigate further. D&D is no exception in this case, it commands the strongest brand recognition by a country mile which I would wager accounts for a large part of it's success. Development of a great set of systems is very valuable and gets players to stick around, but I can't think of any examples of games that successfully sold themselves on the premise of using a certain rule set as their primary feature. The sole exceptions are ones that are direct responses to complaints about wherever D&D is at the time, i.e. Pathfinder 1e or Advanced 5e. Lore, setting, tone, genre, IP usage, all of those seem more important to me in terms of getting attention and initial buyers. I think the reason many of those hundreds of games fail is that they successfully do as I described to an extent, but are ultimately shallow (in some cases barely more than a set of guidelines) or don't have rules that facilitate the experience that was promised; I can recall more than a few 5e compatible or d20 OGL games that didn't mesh rules with the story well at all.