Do you, as a developer, want to wrangle together 20 different payment processors so you can sell shitty 100% polyester T-shirts? No.
Do I, as a customer, want to have to carry around 20 different cards just so I have a chance at being able to pay at an arbitrary merchant? No.
And do I, as a business, want to pay for the additional complexity of managing fees across so many processors? No.
So everyone actually wants the same thing: very, very few payment processors.
Do you, as a developer, want to wrangle together 20 different payment processors so you can sell shitty 100% polyester T-shirts? No.
Do I, as a customer, want to have to carry around 20 different cards just so I have a chance at being able to pay at an arbitrary merchant? No.
And do I, as a business, want to pay for the additional complexity of managing fees across so many processors? No.
So everyone actually wants the same thing: very, very few payment processors.