[random314]

Because there is no clear IVC integration and the integration of Buddhism is rejected by practicing buddhists.

There has been historic and ongoing synthesis which very often follows embrace, extend, extinguish pattern, so the GPs characterization is misleading.

[dhruval]

Buddhism is not mentioned by GP, but there are many big differences between vedic religion and modern hinduism. for eg. animal sacrifice vs vegetarianism.

These ideas probably gain traction from the Sramana traditions which also gave rise to Buddhism and Jainism.

It is hard to tease apart the influences exactly, esp in the case of IVC where very little is known about the culture. But some iconography bears a striking resemblence iconography found in indian religions at a later date.