So, coming back to my original question, why has this practice developed only in India? The wiki page cites mostly religious reasons like ahimsa, asceticism (as meat is considered a luxury), the Law of Karma (you hurt the animals, the bad karma will come back to hurt you), etc. But then, next question would be, why did those religious beliefs develop here? What made people here, and only here, realize that hurting animals is bad? As someone who believes that there has to be an economic reason for most human behaviours, I wasn't willing to accept religion as the only reason for vegetarianism in India. Sure enough, the wiki also mentions economic reasons for not eating meat.
As populations grow, lowering meat consumption worldwide will allow more efficient use of declining per capita land and water resources, while at the same time making grain more affordable to the world's chronically hungry.But that doesn't fully explain the behaviour either, because if the reasons were economic, then you would expect the poorer people to be vegetarian. However, in India it is not so. It is the upper castes like Brahmins and Rajputs (of Vaishnav variety) who are vegetarians by choice. So, maybe it is religion after all which has made us vegetarians.
Speaking of vegetarianism reminds me of an incident which happened a few years ago. I was telling my wife about a team lunch we had gone from office and mentioned that fish was on the menu. My daughter, who was three year old at the time, was surprised and asked "What?". I explained to her that some people do eat fish (and other animals). She said, "No, we shouldn't eat that". I was surprised by her vehemence, because I didn't expect her to have an opinion on anything at that age, let alone on what people should or shouldn't eat. Just to understand where she was coming from, I continued the argument and asked why not. Just as we eat vegetables, they eat fish, what is wrong with that? She thought about it for a few seconds and then said, "But fish have eyes like us. We can't eat them?". I was intrigued by her response. I wondered why did she choose eyes and then realized that she didn't yet have the concept of life or living beings at that age. However, she somehow instinctively knew that fish are more like us than vegetables and when I pressed her she had to think about it and justify her belief. She couldn't say they have arms or legs or head or face, because fish don't have any of those. So she chose eyes as one of the few things they have in common with us. This got me wondering - are kids vegetarians by instinct? Remember, we had not taught her that it is bad to eat animals or any such thing. In fact, that was the first time she even got to know about the option of eating animals and somehow her instinct told her that it is not right. Food for thought.