Question
What caused India to fall so far behind technologically from its zenith in the 12th Century?
Answer
What do you get if you consider a Europe where Christ was born a prince in Rome in 563 BC, and if the Alps were much higher and a few hundred miles further north? You'd get India.
The stories of Europe and India are eerily similar in structure and timing. What Europe had was a more comprehensive destruction at the end of the Roman empire in the 5th century, followed by a Dark Age and a Renaissance that outdid the original. Christianity provided the foundation for the resurrection of Europe, and fundamentally changed the DNA. This large-scale creative-destruction event was central to the achievements of Europe.
India too had the peak of its classical age in the 5th century, during the reign of Chandragupta Vikramaditya (375-413), who could have been the Constantine (emperor, 306-337 AD) of India if Buddhism had come around a little later, and the Himalayas hadn't stopped barbarian invasions from destroying India as comprehensively as they destroyed classical Europe.
So instead of a strong creative-destruction cycle in the first millennium, you had a tired old age for an empire-that-never-was from antiquity, one that basically ran out of steam by 1200 AD and collapsed more from old age than from external forces. What you see today is basically a fading memory, not really a live culture.
Of course, that's only half the story. Europe could have turned out like India if certain conditions had been changed. But that is not the same as saying India could have turned out like Europe if the conditions had reversed. Necessary, not sufficient conditions, is what I am talking about here. Knowing how not to fail and knowing how to succeed are different things. So even if the Himalayas had been less impassable and the Buddha had been born in 1 AD, it is unclear that India would have gone on to have a great renaissance. It may simply have been killed outright. Wouldn't be the first time. Most of the civilizations of antiquity went extinct after all, rather than undergoing renaissance events.
I don't think factors like the lack of a seafaring culture are important though. First, there wasn't as much of a lack as people think (you just have to look at the Hindu/Buddhist cultural layers in SE Asia as far east as the Philippines), and second, it wouldn't have played a decisive role. A powerful naval culture would have been necessary if India had even gotten to certain preliminary stages. It never even got that far. In a way, the operating system needed to be thrown away and rewritten in 400 AD as a basic precondition for other things. The shock required to make that happen never came.
I also don't really think it is useful to analyze Islamic India (1200 - 1750 AD) in the same narrative. Islamic India was basically a large and rich outpost of global Islam, with the center of gravity much further West, in the Ottoman empire.
Finally, I don't think it is accurate to consider India as unfathomably diverse. Yes, it is more like Europe than like a country, but that doesn't mean there isn't a historical continuous sense of self-identity. You only have to read the Mahabharata, Ramayana and the Jataka to get a sense of the subcontinent-wide cultural consciousness that prevailed.
So in short, the question makes sense, the major premise (zenith in the 12th century) is basically correct, and there is a fairly straightforward story that can be told. I could expand this answer out to 100x its length to provide all sorts of supporting detail, historical facts and arguments, but the basic structure of the argument wouldn't change much.
The stories of Europe and India are eerily similar in structure and timing. What Europe had was a more comprehensive destruction at the end of the Roman empire in the 5th century, followed by a Dark Age and a Renaissance that outdid the original. Christianity provided the foundation for the resurrection of Europe, and fundamentally changed the DNA. This large-scale creative-destruction event was central to the achievements of Europe.
India too had the peak of its classical age in the 5th century, during the reign of Chandragupta Vikramaditya (375-413), who could have been the Constantine (emperor, 306-337 AD) of India if Buddhism had come around a little later, and the Himalayas hadn't stopped barbarian invasions from destroying India as comprehensively as they destroyed classical Europe.
So instead of a strong creative-destruction cycle in the first millennium, you had a tired old age for an empire-that-never-was from antiquity, one that basically ran out of steam by 1200 AD and collapsed more from old age than from external forces. What you see today is basically a fading memory, not really a live culture.
Of course, that's only half the story. Europe could have turned out like India if certain conditions had been changed. But that is not the same as saying India could have turned out like Europe if the conditions had reversed. Necessary, not sufficient conditions, is what I am talking about here. Knowing how not to fail and knowing how to succeed are different things. So even if the Himalayas had been less impassable and the Buddha had been born in 1 AD, it is unclear that India would have gone on to have a great renaissance. It may simply have been killed outright. Wouldn't be the first time. Most of the civilizations of antiquity went extinct after all, rather than undergoing renaissance events.
I don't think factors like the lack of a seafaring culture are important though. First, there wasn't as much of a lack as people think (you just have to look at the Hindu/Buddhist cultural layers in SE Asia as far east as the Philippines), and second, it wouldn't have played a decisive role. A powerful naval culture would have been necessary if India had even gotten to certain preliminary stages. It never even got that far. In a way, the operating system needed to be thrown away and rewritten in 400 AD as a basic precondition for other things. The shock required to make that happen never came.
I also don't really think it is useful to analyze Islamic India (1200 - 1750 AD) in the same narrative. Islamic India was basically a large and rich outpost of global Islam, with the center of gravity much further West, in the Ottoman empire.
Finally, I don't think it is accurate to consider India as unfathomably diverse. Yes, it is more like Europe than like a country, but that doesn't mean there isn't a historical continuous sense of self-identity. You only have to read the Mahabharata, Ramayana and the Jataka to get a sense of the subcontinent-wide cultural consciousness that prevailed.
So in short, the question makes sense, the major premise (zenith in the 12th century) is basically correct, and there is a fairly straightforward story that can be told. I could expand this answer out to 100x its length to provide all sorts of supporting detail, historical facts and arguments, but the basic structure of the argument wouldn't change much.