What hindered Ashoka from going into Tibet?

It has something to do with Muslims and Hindus. The short paragraph below is from a website. It is the online article you will find when you look this up.

Explanation:

Tibetan Buddhism has exerted a considerable influence in the Himalayan areas situated along Tibet’s southern border. In Nepal Buddhism interacted with both India and Tibet. Although there is evidence that suggests that the Buddha was born in the southern part of the area that is now Nepal—at Lumbini, about 15 miles (24 km) from Kapilavatthu (Kapilavastu)—Buddhism seems to have been actively propagated only later, probably under Ashoka. By the 8th century Nepal had fallen into the cultural orbit of Tibet. A few centuries later, as a result of the Muslim invasions of India, both Hindus (such as the Brahmanic Gurkha aristocracy) and Buddhists took refuge in the country. The Tibetan influence on the Himalayan tradition is indicated by the presence of Tibetan-style prayer wheels and flags. The Indian heritage is especially evident in the caste system that embraces Buddhists and non-Buddhists alike. In the late 20th century a significant Theravada reform movement took root among the Newari population. The adherents of this movement, who have important connections with Theravada practitioners in Myanmar and Sri Lanka, oppose the maintenance of traditional caste distinctions.