Why do Indian Restaurants have dim light?

The lighting has nothing to do with Indian or International. Its to do with the fact that brightly lit places are mostly fast-food joints (KFC, McD etc.), whereas dimly lit places are mostly sit-and-dine-and-a-waiter-will-serve-you kind of places. Both are restaurants by the way. At the same time let me tell you that many Indian restaurants like the Amul restaurants, Barbeque Nation and Haldiram’s are all brightly lit.

Dim lighting is generally preferred because it is more romantic, attracts more affluent clientele and thus higher prices, sets the mood, and it also hides the fact that you are eating plain looking food. Also the crowd is quieter. Such are the places that you will go to if you want to have a quiet dinner with your family.

Bright lighting is preferred in fast-food joints because it promotes energy, activity and attracts all kinds of people, being much more populist on nature. Such places promote enjoyment, loud conversations and a general celebratory attitude.
So, it ultimately boils down to how a restaurant styles itself and who its clientele are.