Where are you based? When it comes to the East I have noticed that Western historians focus a lot more on the Far East (China and Japan) than other Asian countries. India's journey was intrinsically tied to that of the UK for a couple of centuries - if you are elsewhere it may not have made much of an impact. Ancient India had a tremendous impact on the world. Starting with wiki is a good bet, pm me if you have any particular questions.

junesponykeg gave the best answer as to why you specifically haven't learned about it, but I'll give a brief overview as to their importance in history from a western world perspective. Remember that significance in history is entirely about perspective.

India's ancient and medieval world importance to the west comes mostly from long-distance trade. Recall that Christopher Columbus was not originally aiming for America, but India, since the Europeans enjoyed commodities from the region but had no direct trade routes to get it.

Once Imperialism was viable for European nations, India became a sought after prize. Many nations had pieces of territory on the subcontinent, but eventually it was Great Britain who came to dominate the region. For them, India was an enormous boon; it provided them raw materials with which to fuel their industries (Great Britain specialized in textiles) as well as opium which they could trade to nearby China for an excellent profit.

Calls for Indian independence were finally met shortly after WWII, with Mahatma Gandhi being the most prominent figure of the movement. Originally the proposed Indian nation encompassed the entire subcontinent, including the today independent nations of Pakistan and Bangladesh, as well Sri Lanka (also known as Ceylon). They were largely neutral in the Cold War, as they were a founding member of the Non-Aligned Movement.

EDIT: Accidentally a name.

When I've asked my high school and college teachers in the past, the most common answer is that it is simply too complex and time consuming. Especially on the topic of Hinduism, with all the deities and complexities it would take a semester or more just on that. Also, if you live in the western world, the history class you would take would presumably have a western focus. Outside of trading, India, and to a lesser extent China, did not have much of an impact on the western world.