Great information. I made a very general analysis, with the aim of covering the majority of territories and populations, not of exhausting every local specificity. Hinduism is not a uniform religion, as i said, but a very diverse one. But still, there are common traits that unify the vast majority of Indians that makes the situation very different (less diverse) compared to the historical European polytheism. The vast majority of Hindus believe that religious knowledge and rituals must be studied and performed by a specific social group, the brahmin caste. Or will you tell me that a random foreigner or a Dalit can enter any temple in north or south india, study sanskrit and rituals, and start preaching and performing rituals ? Not by a long shot. Hinduism and local religions have a diversity of holy texts, but the important aspect is that in each tradition and place there is a good degree of codification and formalization: at least one set of written (fixed) texts that people will adhere for doctrine and rituals, not for instance an exclusively oral tradition that changes radically in each house of worship, over time, and over the next village (old polytheism). This is stuff that only a developed urban civilization makes, that makes a religion have more 'capital' so to speak (to spread and be reproduced over time).

The muslim rulers also did not immediately convert everyone to islam in the conquests over persia to north africa, but due to the characteristics of traditional polytheism (along with the conquests and violence), Islam converted and spread over time, including to places christianity had not reached (like interior Egypt). This did not happen with indian religions because of the higher degree of formalisation and codification, that allowed it to at least keep up ideas and rituals with a more equal power degree in syncretisms.