Lots of players would rather to use only a limited number of systems.
D&D is the master of the market and the brand who created the hobby. But they aren't ready to be an universal system for all the genres. Of course a d20 sci-fi game is possible, but I talk about being totally compatible. The martial artists from Street Fighters can't face the heroes of Overwatch, the champions of Mortal Kombat can't fight against the infernal monsters from "Doom".
The settings are an important part of the brand, but D&D is not only the settings. Lots of players would rather to create their own worlds, some times mixing from different sources. For example somebody could use the Ravenloft setting to play a version of Innistrad to play with the factions from World of Darkness+Chronicle of Darkness.
WotC is selling the bricks, but you are free to build the same building than in the cover of the box or a different thing.