Before we begin, let me explain some background information. Once Muhammad and his followers (the umma) made Islam the majority religion in Mecca, his followers agreed to spread Islam. One of the principle ways in which religion spreads is through trade routes [for other examples of this, think about Christianity and Catholicism on the Atlantic Slave Trade, or Buddhism spreading from India to China on the Silk Roads].
Using the nomadic Berbers on the Sahar desert, Muslims engaged in a jihad across Northern Africa. This crusade converted much of the people in North Africa. When it comes to Western Africa's conversion to Islam, there were two factors. The first was the spread of Islam across North Africa; it made Islam more accessible to the West African kings. The second reason is a bit more complex; let's use Mansa Musa as an example.
Mansa Musa was king of the Kingdom of Mali. While his state was an example of a centralized monarchy in classical Africa, he was interested in Islam. By converting his people to Islam, he would be able to create a commercial link to the Middle East; Muslim merchants were the principle traders throughout this period. Having a strong economy based off of trade with the Middle East would also be political justification for his reign, and his empire was consolidated and centralized.
To recap and answer the question, West Africa was influenced by Islam by a greater commercial link to wealth of Muslim merchants. Once Western Africa was converted to Islam, its leaders and people adapted Islam into West African culture. While certain local practices and traditions remained in West Africa, Muslims in the region followed the five pillars. Mansa Musa himself went on several pilgrimages to Mecca (the hajj), and each time brought with him a display of opulence and wealth to help establish power for the Kingdom of Mali. He even commissioned one of today's most fascinating mosques (in my opinion), the Great Mosque of Djenne.