To clarify: Buddhism did continue on its ascendency for over a millennia after the time of Emperor Ashoka (~ 300 BCE). Buddhism was likely the dominant religious force in Northern India, more or less, till about the 6th century CE, while Southern India was a stronghold of Jainism. The Kushans, Indo-Greeks, Indo-Parthians, Sythians, and the Huns all tended to patronize Buddhism. It was only with the Guptas (mid third century CE) that a major empire favoured Brahmanical Orthodoxy. Even with the Guptas, it seems likely that the later emperors leaned towards Buddhism, while officially stating devotion to the family deity, Vishnu.

Even as late as the Emperor Harsha (~ 600 CE), Buddhism was widespread in Northern India, and the Chinese traveller Xuanzang attests to this. Harsha's own brother and sister were Buddhists, while his court poet notes Harsha as a devout Shaivite. Xuanzang records Harsha as Buddhist, and describes festivals where the emperor worshipped images of the Buddha. Xuanzang writes how Harsha would organise a charitable donation every five years, where he gave generously to holy men of all orders. Of the 21 days marked for charitable donations, four were for the Hindu holy men, while 16 were for the Buddhists. Does that suggest Harsha's leaning towards the Buddhists, or was this a consequence of the large number of Buddhist monks? Unclear.

What these and other sources indicate is that Buddhism was alive and well in Northern India, almost 1500 years after the time of Ashoka. We do know that shortly after the death of Harsha, Buddhism was in deep decline across India. The reasons are unclear. It was likely a combination of multiple factors, some of which include:

• The bad blood between the Theravada & Mahayana schools had gotten especially heated by this time period. Xuanzang notes a gathering of Buddhist monks across various sects called by Harsha to discuss points of theology. The gathering ended with some disgruntled party torching the venue. Even if this is hyperbole, it underlines the animosity that existed between the rival schools.
• Monasteries had grown isolated and aloof from the communities they were in. In the early centuries of the monasteries, the monks were dependent on the laity for their sustenance. The lay folk would travel to the monasteries to care for the daily needs of the monks. Over time, monasteries became self sufficient on royal grants of land. As a consequence, the lay folk of the surrounding communities grew less and less invested in the welfare of the monks.
• Hinduism had continued to develop theological sophistication since the advent of Buddhism, and offered a muscular philosophical response to Buddhism. In the intervening millennia, Hinduism had developed the Upanishads, the Itihaas texts of the Ramayana & the Mahabharata, the Puranas, and Vedanta. Buddhism had continued to evolve as well, though it would seem that those efforts were aimed at rival sects. In addition, the hindu bhakti movements gained traction among the public, further weakening Buddhism's appeal.

By the 11th century, Buddhism had practically disappeared from North India.