Because of the taste? While it’s not common to brew a drink with other beans, we eat them all the time, and it’s pretty obvious in doing so that they aren’t flavors that lend themselves to a beverage.

Coffee beans are actually the seed of a more traditional “fruit” (ie, sweet and acidic) rather than a legume like other beans (also technically seeds, but vegetal in flavor, with an entirely different taste and texture). You’re basically just going to get a weak broth from traditional beans.

Similarly, people have tried steeping every type of leaf, plant, and fruit out there in water, but it’s a pretty limited list that remains popularly used for tea, as it’s a pretty limited list (relative to the incredible diversity of plant life) that actually tastes good that way.

People use mushrooms, various roots (like chicory), other fruity seeds, and more to create coffee-like drinks, and/so with the number of people and cultures out there with their own tastes and traditions, it’s a relatively safe bet that if people aren’t drinking it anywhere in the world, it’s because they’ve tried it and it just doesn’t taste good.