The Himalayas are a HUGE part of what prevented this. It prevented Chinese invaders and Indian expansion. The mountains of the Himalayas reached record heights and were almost impossible to traverse. The snow slowed travel, there was no food, so hunger became a problem, and the air was thin at the high altitude, so it became harder to breathe. Not to mention the obvious, but it was extremely cold as well. Anyone who dared climb it would return frostbitten.
Hope this helped! :D