What was the causes for downfall …

The Mauryan empire declined on account of the following reasons:
(i) Incapable Inheritors: After Asoka, the reign went in the hands of rulers like Dasrath, Samprati and Brahdrath. All these rulers were incompetent.
(ii) Vast Empire: The Mauryan empire had witnessed a phenomenal expansion during the reign of Asoka. However those who inherited his legacy proved to be very weak rulers. They failed to rule it properly.
(iii) Lack of Law of Succession: There was no specific law of succession in the Mauryan dynasty. So as and when a ruler died, the princes started fighting against each other to get the way to throne. The Mauryan power became very weak due to such civil wars.
(iv) Internal Incohesion: After the death of Asoka, there was an internal rebellion in the empire. Many provincial governors had declared themselves as independent. Consequently the Mauryan empire started crumbling.
(v) Lack of Money: Money is very important in the governance of a state. But Asoka spent generously in preaching and spreading Buddhism. He also spent a lot on public welfare activities. As a result, there was no money in the royal treasury. It loosened the grip on the administration. The subsequent rulers failed to nip the rebellion because of lack of money in the treasury.