You can perceive that the games that sell the most are the ones that can give the most instant gratification to the players. It includes all those Free-To-Play shit on mobile, Call Of Duty, Mario, Wii Sports, GTA 5 ( it does have a complex system to learn, but most people only go destructing everything) and etc.
More brain games are more difficult for the general public to appreciate, or either they have a very big hardcore fanbase that can push millions of units, like MGS, this franchise always sells around the same numbers since PS1, FF, DQ, Blizzard games and MMO`s in general aren`t for the general public, but they happen to have such a huge fan base that they look to not be niche, but they actually are.