It never managed to spread in China while Buddhism did ?
Let me answer you in proper religious context. Give me a big "Thanks" and close thios thread thereafter
Well you see, Lord Buddha is the 10th Avatar of Vishnu. It is considered to be an "Aveshavatar". That means Avatar happening under influence of "Avesh" or rage.
It is this avesh that prompted Lord Buddha to Bad mouth Vedas throughout his prophetic career. As can be seen that people like B R Ambedkar who disowned Hinduism, promptly converted to Buddhism, because buddhism fundamentally disagrees (although mildly) with sosme basic principles of Vedas. It is understood that prominence of Vedas is divine and eternal and guaranteed by Brahma-Vishnu-Mahesh. (The creator, the protector and the destroyer)
His Holiness Adi-Sankaracharya explains that due to Lord Buddha's friction with Vedas, Buddhist religion will never flourish in those parts of the world where Vedas have had prevalence. That is the reason in main land India, buddism has not been much prevalent, even though the we had a newer local prophet. But it prospered outside Indian mainland, viz srilanka, china, east asia, Afghanistan etc.
Aso Adi Sankara was himself an Avatar, that was made to counter the rising buddhist tendencies. People attribute his Incarnation as an upkeeping of Lord Krishnas Vow ("
Yada yada hi Dharmasya glanir bhavati bharat".. fame). Adi Sankara was solely responsible to check and overthrow rising buddhist tendencies in mainland India.
At the small age of 31, this great great soul had defeated thousands of buddist scholars in intellectual Shastratha, as a result of which all those defetaed buddhist scholars had to end their life as per rules of Shastratha, by accepting that Vedic Dharma is supreme. Therby putting an end to rising buddhist tendencies and promoting of Indian core dharma. Sankaracharya himself went into infinite meditation and took Samadhi at the age of 31 after completing his mission and establishing 4 Shankaracharya peeth in 4 corners of India.
My personal belief is that Dhrama is eternal and immortal and in guranteed protection of the Supreme. That is why almost all places that recent barbaric desert kabila religion has been to have almost completely converted. Except for India. And it will be like this forever.