Manichaeism is a complicated subject. A while back I wrote on it here. It can be regarded as a syncretism of Christianity, Buddhism and Zoroastrianism.

What I would add now that I was less read up on then is that Manichaeism fits into a larger pattern of syncretisms that include Zoroastrianism and Buddhism, along with Hellenic religion and Mesopotamian religion. Such syncretisms flourished in the Indo-Greek Kingdoms and under the Kushan Empire in the centuries preceding the Sasanian conquests and Mani's rise, and they may well date back to the Achaemenid era. From this perspective, Mani wasn't necessarily so unusual.

On the particulars of its decline, I don't feel qualified to comment.