After finishing Escape from Rome: The Failure of Empire and the Road to Prosperity I continue to believe that geography and economics explain the basic reason for the very long ascendance of Turkic people in the Indian subcontinent, and, their eventual eclipse.
The context for this is the fact that many Indian and Indian American friends have posited cultural rationales for the Turkic hegemony. That is, there was a passivity and disunity in Hindu society which made it vulnerable to the Turks, who were also adherents to a separatist hegenomic ideology in the form of Musim. All this may very well be true, but I have always held that the key factor was that Turks and Muslims more general had ready and easy access to warhorses.
Between 500 A.D. and 1500 A.D. was the heyday of the nomadic pastoralist as a geopolitical force in Eurasia for various reasons. Even in the centuries after 1500 A.D. horses remained critical for mobility. The problem is that in much of agricultural Eurasia there is not sufficient pasturage to raise large numbers of horses.
Pretty much every Eurasian society within reach of the steppe (so basically every society except for those in Western Europe, Japan, and Southeast Asia) had to deal with the menace of armed nomads. Sometimes they paid them off. Sometimes they mobilized enormous armies which incurring crushing costs. And sometime they were conquered.
Between 1700 and 1900 this spectre faded. Improvements in military technology, transport, and mass national mobilization, leveled the gap between the steppe and the settled peoples, to the point where by 1900 the steppe was a marginal factor.
Note: the author of Escape from Rome attributes the rise and fall of Vijayanagara explicitly as a function of its access to horses.