In 1269, Marco Polo's father Niccolo and his uncle Maffeo, returned to Venice from the far east. They carried with them a letter from Kublai Khan, for the Pope.
Kublai Khan had conquered all of Asia and wanted to unite his empire. His problem was, that all the different provinces were always at war with one another. He knew that he needed some kind of civilizing influence to put an end to the constant conflict among them.
While Niccolo and Maffeo were in his court, Kublai Khan questioned them about Christianity and he realized that this could be the civilizing influence he needed to bring peace to his empire. So he wrote a letter to the Pope, requesting 100 missionaries to be sent, who would travel around Asia and teach his people about Christ. He asked Niccolo and Maffeo if they would deliver it to him on their return to the west.
When they returned to Europe however, Pope Clement IV had passed away and they had to wait for two years until Pope Gregory X was elected. In fact they had grown impatient and taking the 17 year old Marco with them, had begun the journey back to China, when news came of the election of the new Pontiff. So they returned to Rome with the request.
Pope Gregory was not prepared for his election, as he was involved in the Crusades in Palestine. He was so focused on the Crusades and needing men for them, that he was not interested in the far east. Subsequently, his compromise was to send two Dominican monks in response to Kublai Khan's request.
The two Dominican monks were used to the comfort and security of the monastery and when the journey through the mountains became too difficult for them, they turned around and returned home.
Kublai Khan then had to look for an alternative civilizing influence to bring peace to his empire and news came to him of a relatively unknown religion in the mountains. He saw that the Buddhists had a way of life that he believed could be the civilizing influence he needed, to bring peace to his empire.
This is how one man's misguided sense of priorities and the lack of resolve in the two monks, led to the spread of Buddhism across all of Asia, instead of Christianity.