Explanation:

The Guptas with their seat of power in Uttar Pradesh and Bihar ruled over north and western India for about 160 years, till the middle of the sixth century A D. Then north India again split up into several kingdoms. One of the dynasties ruling at Thanesar in Haryana extended its authority over all the other
feudatories. The ruler who brought it about was Harshavardhana (A.D. 606-647).

Harsh made Kanauj the seat of his power and therefrom extended his authority in all directions. During this period Pataliputra fell on bad days and Kanauj came in the forefront.

Reasons for the decline in significance of Patliputra are:

Pataliputra owed its power and importance to trade and commerce and the widespread use of money. Tolls could be collected from the traders who came to the city from the east, west, north and south by means of four rivers. Harsha did not mint many coins and used to pay the officers through land
grants. Once the money supply became scarce, trade declined and the city of Patliputra lost its importance.

Kanauj was strategically located which could be used to dominate long stretches of land. Fortification of places in the plains was far more difficult, but Kanauj was situated on an elevated area, which was easily fortifiable. Located right in the middle of the doab, it was well-fortified in the seventh century. So to exercise control over both the eastern and western wings of the doab soldiers could be moved by both land and water routes.

Many historians have attributed the flooding in nearby rivers to be one of the causes of the decline of the Patliputra. Since it was located on the banks of the river, it suffered floods from rivers like Ganga & Son. Reference to such events has been cited in the text of Chinese travelers.