The King of Mali was converted to Islam because he met with Muslim merchants and considered their technological and logistical knowledge as indicative of the religion's merit. Mali was like most pre-modern states, in that once the king or tribal chieftain was converted to a religion, the population was compelled to follow suit.
It helped spread the news about Mali to Africa and got more people to go to Mali. Also, his pilgrimage/hajj showed that he was a devote Christian and wanted to get others to convert to Islam, too.
During Mansa Musa's reign, Islam became very popular in Mali. Following their king's example, many people from Mali went to Mecca. In turn, many Muslims from Asia, Egypt, and other parts of Africa visited Mali. These journeys between regions helped create more trade and made Mali even richer.
Prophet Muhammad spread the religion of Islam so that people can follow/practice this religion. Just like why did Jesus spread the news about Christianity?
some of their belief was that they believed in respect ancestors and so on
It is difficult to estimate who played the most important role in the spread Islam in the sub-continent. There are so many Scholars and Sufia who spread Islam in the sub-continent.
Prophet Muhammad (peace be upon him) taught his family and companions about Islam and then the word went around and with the conquests , empires, and converts spread to the entire earth.
The reason why Islam was so successful is because it was spread as soon as the message was given to the prohet Muhammad(PBUH)
The greatest king of Mali, Mansa Musa returned to Mali after traveling. He returned with an Arab architect who built great Muslim houses of worship , or mosques, in the capital of Timbuktu. Under Mansa Musa, Timbuktu became an important center of Islamic art and learning.
so that there would be more people to join the muzlims on there journey to the holy place
The Arabs spread Islam by conquering the people around them. Then they put high taxes on non-muslims. They beheaded a number of people worshipping idols. So, they used a mixture of techniques.
It was a fusion of Islam and traditional African religion - religion that involved magic, animism, etc. Even when Islam dominated, aspects of tribal African religion remained with legends of genies and accounts of magic.
As Islam spread throughout middle eastern lands, it changed. It became differed with each region it came into, creating a localized religion. Eventually it became so different it split into 2 caliphates