Familiarity, the quality of being known from past experience, might be the biggest reason why many people stick with just one roleplaying game. Our sense of acquaintance with a set of rules, our intimate previous knowledge of how things work, is an appealing draw whenever we are wanting to play a game.
Friends have been commenting on how the increasing number of games being published for 5th Edition which essentially repackage already existing worlds and universes are, to their minds, “obvious cash grabs”. I am not so sure.
My feeling is that 5th Edition is a trusted an popular fantasy RPG with a colossal following. Looking at it from the perspective of someone who already plays D&D, it seems reasonable that if you are (say) interested in Middle-earth, Pulp Superheroes or Doctor Who, then you might prefer your rules to be familiar. It saves learning something new. It avoids the risk of “I might not like this new rules set”.
Yes, companies publishing for 5e have a huge market. They are also servicing a need for people already in that market. The rules are familiar and safe for those who play D&D already. This makes providing them with a new setting, even a new genre, less intimidating. It’s convenient and will feel reliable.
Meeting people’s needs is a good thing. It might not be my need to have a 5e version of whatever but it’s not necessarily merely a cynical cash-grab to publish it. Maybe familiar rules will be a doorway to new adventures in other realms.