The development of Indian metaphysics had its most formative and creative period during a time of political ferment.
In the period from the fall of the Mauryan empire to the rise of the Guptas - i.e. from about 200 b.c.e. to 300 c.e. - the various parts of the Indian subcontinent were ruled by a number of diffeent empires. There were the various foreign powers such as the Bactrian Greeks, the Shakas, the Pallavas and the Kushan empire in the north-west (all of whom were assimilated into the Indian culture and converted to either Hinduism or Buddhism); and a number of native dynasties such as the Shungas and the Kanvas in the north, the Andra Satavahanas in the central-south highland plains known as the Deccan, and the various Tamil dynasties in the south.
During this period belong the two great epic poems - the Mahabarahta and the Ramayana - in their final form, as well as the minor Upanishads, some of the Puranas, and some of the philosophical Sutras. It was during that period that Mahayana Buddhism arose in the north-west, and its main philosophical schools developed [D. S. Sarma, Hinduism through the Ages, p.20].
By the end of this period, the Upanishadic speculations of the first millenium before Christ were finally codified into the various Indian (or "Hindu") philosophical systems. Now, whereas in the "heterodox" Indian traditions of Buddhism and Jainism, philosophy and religion form a single gestalt, in "Hinduism" the philosophies co-exist quite comfortably with all the religions (Vaishnuism, Shaivism, etc).