To understand the religion of India you would have to understand why there are many flavours and schools. Thus the question here is not really answerable in an internet post but only by following the teachings and acquiring knowledge. The answer by Ted Wrigley covers some of the ground but there is more.
A number of factors are in play.
First, all the sciences are surrounded by a halo of misunderstandings and pseudo-science and especially where the science is subtle and complex, as with QM for instance. This is true for all religion. If we examine modern science we find there are people who believe the earth is flat, that relativity is a Jewish conspiracy and that the theory of evolution is the work of the Devil. Yet we do not ask why there are so many sects in the natural sciences. We study the subject and soon learn to distinguish the good stuff from the nonsense.
Second, generally-speaking Indian religion teaches that the true nature of reality is beyond conceptual fabrication. This means that while we can point to it we cannot describe it, and this leads to ten thousand misunderstandings. Thus those who have not realised truth must rely on speculative interpretations and these may vary considerably. Meanwhile the truth would be beyond imagination and only available to those who have done the experimental work.
Third, as you dig into the heart of all these sects you'll find that they begin to merge. Often they are different approaches and methods, for each of us has our own personality and preferred approach, but not mutually inconsistent doctrines.
The crucial distinction between religious teachings is between those that help us discover the truth and those that tell us what to believe. The latter approach is not popular in the Indian tradition, although even there many believers are happy to believe rather than investigate. .
Fourthly, the nature of Reality would be such that our language cannot convey it. You ask why teachers do not explain more clearly the nature of reality, but once one grasps what it is they are saying it can be seen that they could not explain any more simply and clearly. They are doing their best to explain simply and clearly and often succeeding, but it will not appear this way if we do not understand what they are saying. The language of Unity and non-duality is non-ordinary and easily mistaken for a muddle, but once 'grokked' it is simple and clear.
Fifthly, people will believe any old thing if it suits them. To compare religions, sects and schools it would be important to dig into them and find the expert writings and teachings, for these will always be surrounded by the dodgy views of students and muddled scholars. But their correct interpretation cannot be discovered from merely reading texts, just as we cannot interpret the word 'pain' by merely reading a book.
Sixthly, religious teachings are a game of Chinese Whispers. As time goes by they become subject to various different readings. In my view Roman Christianity and its departure from the original teachings would be a paradigm case.
There are sects in Buddhism, notably Therevada and Mahayana, that disagree quite violently. In the second-century Nagarjuna demonstrated a logical proof of reality in order to settle these disputes and it should have worked. But even all this time later the Theravadans won't budge. People will believe what they want unless they are truth-seekers. Philosophers have the task of sorting the wheat from the chaff. It can be done, but only by abandoning speculative interpretations and ungrounded beliefs for logic and experience, which is what most the mainstream Indian sects advise us to do.
