Ultimately the question of why the Han ethnicity is the largest in the world (by far) is a complex question that will never have a straightforward answer.

China is a fairly well-defined geographic region which supports a massive population. If we compare to the Nile, the Tigris, and Euphrates, it is fairly straightforward - China just has a bigger population than Iraq/Mesopotamia or Egypt. However it is hard to answer definitively why geographic regions with similar populations like the Indian subcontinent or Europe have fragmented into smaller ethnicities while China has remained relatively ethnically homogenous.

There's a couple of different angles where you could approach this question - one of them was discussed here.