I am a Hindu by birth and I can tell you that, it is a very liberal religion, and monotheism seems to have wiped out every other major religion on Earth except Hinduism. This is what surprises me, because Hinduism is not a missionary religion and from my deductions should have been easy converts for the missionaries. But as seen today, this did not happen. What was the reason?
You seem to be asking 2 different questions. 1.) "Why were Islam and Christianity unsuccessful in getting converts in India" and 2.) "Why wasn't Hinduism completely eradicated by a monotheistic religion." These are two different things, and let me first say that Historians can't exactly explain why something didn't happen. That's not within our capacity, but I'll gladly share some relevant insight with you.
For one thing, your first question is based on a bad premise. Islam and Christianity were both successful in penetrating India. As of today, roughy 1/3 of the global Muslim population either lives in the Indian Subcontinent or is of that heritage. Indian subcontinent because, as Im sure you are probably aware, prior to the British Raj there was no cohesive Indian nation. Just a sub-continent of various independent polities. While they most likely had a cultural understanding of being a part of the Hindustani fabric, they were all in practice nations independent of each other. Even when united under one of several Empires such as the Maurya of Mughal, these nations would not have seen each other as the same people. Prior to the advent of Islam or Christianity, these would all have also fallen under a Dharmic tradition- either Hinduism or Buddhism. Anyways, fast forward a couple thousand years, and the modern States of Pakistan and Bangladesh (which emerged from 2 of India's most prominent states) exist at all because of how deeply Islam penetrated India. And even with that being the case, the Indian Republic is still most likely home to the 2nd largest Muslim population in the world. I can't see how Islam failed to penetrate India with these things being the case. As far as Christianity, while it is less prevalent in India, so too is the amount of time that India was under rule by Christians significantly less than the time India was under the rule of Muslims. Even then, there are states in East India today that are more Christian per capita than the American South, with Christianity being the 3rd largest religion in the Indian Republic. So, again, i would stop short of calling that 'unsuccessful'.
But this is a good point to pivot to your next question. Why didn't Hinduism get eradicated completely? Truthfully, I cant answer this question. Dont know that anyone can. But for one thing, look at how despite being home to so many Muslims (almost half the Arab World even), the Muslim population of the Indian Republic is only 14% of the overall population. INDIA IS HUGEEEEEEE. Both geographically and demographically, and naturally this has had an effect on how thoroughly these lands have been proselytized. In fact, there is only one macro-region with a comparable population, China, and that region mind you hasn't totally adopted monotheism either.
Another point of consideration though, is that Hinduism was in a sense created by the British. What I mean by this is, prior to the Raj, there was no centralized or universal 'Hindu' religion/identity. There still isn't. Rather, there were hundreds of Dhramic traditions that were indigenous to the sub-continent, traditions that would vary wildly from region to region. The British took all of these and compartmentalized them under a catch-all term: Hinduism. Prior to this, there really was not a universal Hindu identity or sense of nationhood, going back to the earlier commentary on how the Indian states saw each-other as different nations and peoples. If there was, a tiny Muslim minority would not have been able to rule the 'Hindu' super-majority for so long, no way. Rather, the Islamic rule of India as well as the spread of Islam in India happened at a state by state basis. No one went in looking at the whole of India as one nation or land to conquer/convert but rather a region of a bunch of disjointed nations (and rightfully so). Contrast this for example with the Ottoman incursions into Europe, which was rebuffed with a united Christian front. It was understood that that was a Muslim vs Christian ideological alongside the physical conflict. This was not always the case in India.
For this reason, there wasn't a campaign or push to conquer/convert India all at once. That would have been akin to the notion of trying to conquer/convert "Asia" or "Africa" in one breath, simply unheard of. So as a result of this, we can see that Islam spread more or less in an unorganized manner from state to state, 'nation to nation'. SO my point is, if you look at it in terms of "the spread of Islam in Bengal/Punjab/Sindh/Hyderabad/Tamil/Kerala" as opposed to "the spread of Islam in India at large", you have a better framework of understanding how the Muslim conquerers themselves saw it. And in this regard, some nations/regions/states were fully engulfed by Islam. Its all about perspective.
Im sorry if that doesn't fully answer your question, but I don't think there is any single answer to why Hindusim wasn't completely eradicated in India. But in summary, Im sure the incredible size and scale of Hindustani civilization was a factor. But alongside the point about perspective, keep in mind that both Islam and Christianity in fact did successfully penetrate India, at least to some extent.
Sources:
Inscribing the Other, Inscribing the Self: Hindu-Muslim Identities in Pre-Colonial India Cynthia Talbot Comparative Studies in Society and History Vol. 37, No. 4 (Oct., 1995), pp. 692-722 Cambridge University Press
Who Invented Hinduism? David N. Lorenzen Comparative Studies in Society and History, Vol. 41, No. 4 (Oct., 1999), pp. 630-659 Cambridge University Press
Pennington, Brian. The Invention of Hinduism: Britons, Indians, and Construction of Religion in Colonial Bengal. New York: Oxford UP, 2005. Print.
King, Richard. Orientalism and Religion: Post-Colonial Theory, India and "The Mystic East" New York: Routledge, 1999. Print.