Well, chilli grows far more easily than black pepper, and consequently is far more affordable, even today. Secondly, it takes far more black pepper by weight to induce any kind of burn. Thirdly, they taste very different.
The widespread use of chilli as a spice in Indian cooking would also have an economic aspect to it, I'm sure.
Fun fact: the one place (speaking for Southern India) where you're likely to find cooking that uses only indigenous (pre-chilli -- ergo pre 16th century) ingredients is any kind of Hindu religious ceremony or celebration associated with death, be it a funeral or a death anniversary. That stuff tastes OK, some of it is great. All told, I'd rather have food tainted by "modern" ingredients than not.
Edit: qualified the "celebrations and ceremonies" bit
The widespread use of chilli as a spice in Indian cooking would also have an economic aspect to it, I'm sure.
Fun fact: the one place (speaking for Southern India) where you're likely to find cooking that uses only indigenous (pre-chilli -- ergo pre 16th century) ingredients is any kind of Hindu religious ceremony or celebration associated with death, be it a funeral or a death anniversary. That stuff tastes OK, some of it is great. All told, I'd rather have food tainted by "modern" ingredients than not.
Edit: qualified the "celebrations and ceremonies" bit