Upvote:6
We don't have any decipherable native writing from the subcontinent until the 3rd Century BC. This means anything we know about what was going on in the area prior to the Maurya Empire has to be derived either from the records of neighboring civilizations, or via archeology.
Greek accounts are particularly useful during the periods of time that Greek armies were in possession of portions of the Indus Valley (about 326-70 BCE). This is because Greek histories were highly-prized in the West, and thus widely copied, which means many of them survived to modern times.
The Chinese tended to have a strong interest in the goings-on in neighboring areas beyond their borders, based on a healthy sense of self-preservation. They also held great reverence for their histories and, helped out by their relatively continuous culture, many of those records survived to modern times as well.
It appears that even when Indians did develop native literature, most of their preserved manuscripts seem primarily interested in things like (Hindu) Philosophy, poetry, and Grammar. All worthy priorities for preservation to be sure, but History does not appear to have been among those priorities.
This is sub-optimal of course, and most of the local details not available in outside accounts have to be filled in via the tools of archeology. But we work with what we have.