Without the advantage of its wide familiarity, most people would play something else besides D&D.
There. We stopped.
I disagree.
D&D has been in print continually for 50 years. It's had multiple different editions, with considerable variation in mechanics and playstyle, yet for each of those 50 years (barring potentially a short period in the 90s vs Vampire and potentially a short period in the early 2010s vs Pathfinder) D&D been inarguably the most popular RPG, by a considerable margin. Whatever else was going on in the industry, whoever owned D&D, whoever wrote D&D, whoever drew the pictures in D&D, it was always (or practically always) number one.
Your position is that each one of these different versions and editions of D&D happened to be the best designed, written, and produced RPG of its era?
Or could some other factor have contributed?