is simply that India is big and the initial invasions that took Delhi and imposed Muslim control over the Indo-Gangetic floodplains had a lot to digest.
Indeed over the next half millennium Islamic warlords did progressively set up shop further and further South. The thing is by that time they were already coming South as Indianised warrior aristocracy- the process of conversion had been freighted with a lot of caste baggage.
I think this is broadly true, but a couple of things occur to me: You still have, and had, plenty of Hindus in the territories ruled by Muslim dynasties for the longest period of time; and explaining that the caste system made conversion more complicated seems like an explanation for why Hinduism.
Charlemagne stepped it up to imperial policy by demanding conversion or destruction from a number of neighboring petty states in the years after the Saxon Wars which was continued under the early Holy Roman Empire. In some places (Scandinavia, the Baltic) this process got pretty violent. Ultimately the offer of trade, power, and privilege in the eyes of Western Europe won out for most elites and when the guy with the sword is killing anyone who isn't going to mass the commonfolk toe the line sooner or later.
Sure, but there were also states and kings which rejected Christendom because they saw it as an attempt by European powers to subvert their authority, and anti-Christian beliefs were a sign of resistance; this comes up not just with the Saxons, but also the zoroastrians in early Islamic Iran.