A customer that knows what he wants and doesn't accept only one option for his favorite rulesystem. Using an analogy, I'd rather see many varieties (brands) of peanut butter on the shelves, even though they're based on a basic peanut butter recipe. I don't want to see only one choice.
True enough, but do you expect the customer to buy everyone of those brands of peanut butter or just one. I like variety as a customer, but in the end it is just another brand of peanut butter (personally, I like it whipped
).
But, as a publisher, I am going to cater to the rule system that sells the most and not devote my time to off-shoots. Furthermore, I will do it by not rehashing their elements and selling it as new, but adding components to it hopefully improving the game.
So, using your analogy, I cater to those that eat peanut butter (the most popular brand; i.e. those in the majority) and not cashew butter, or "hey, it tastes just like peanut butter", or the supermarket brand that uses the same recipe.
It's not good business sense.
On the flip side, as a customer, I like delving into new systems: Deadlands, Vampire, etc. or seeing how others tweaked the 3.X rules system to make a new game (True 20, Iron Heroes) - I might borrow from those systems - but in the end I am still playing good ole' D&D on Friday nights (and so does a majority of gamers).
So, why, as a publisher, should I devote my time rehashing what is already done for just one or two customers to have variety when I can make something entirely new to fit in the system they already love and enjoy.
Fun for a customer, but not great business sense as a publisher.