I would say that Islam plays not just somewhat of a role but by far the largest role. Islam is a large world religion with a long history, a vast literature, and a huge international religious, cultural and even military and economic network. Furthermore, without straying from the topic of history to theology, like other Abrahamic religions it takes a very firm and explicit stance on its own absolute truth rather than allowing for the degree of relativism common among animistic religions. Islam was from the very start more than aware of the existence of Christianity, and had formal responses to it. Those religions that had not had contact with Christianity up to that point did not, and Christianity could be introduced afresh, and even as an alternative that allowed for some degree of syncretism: yet another god, and one whose people clearly had the backing of a relatively far more advanced material culture. The particular paths in which missionary work and conversion took place depend greatly on the particular area of Africa - the story of arguably self-destructive Xhosa millennialist movements that led to failed last stands against the British to the gradual syncretism that took place between Vodun and Christianity, for example, are quite different. An organised attempt to convert the Muslim populations at a grand scale - not that there were none - would have resulted in a far more organised and greater backlash, and been less effective per person.

The other factor is material: the network and incorporation into the Islamic world, and a long history of advanced civilisation before that, also corresponded strongly with greater material development for North Africa than most of the continent. There were both Islamic and non-Islamic civilisations in West Africa during the medieval and early modern periods, but the former were further north - ie, inland - and it was the latter that were exposed to greater European contact and for longer. Even Muslim civilisations of the interior had a well established Islamic high culture, with the famous university of Timbuktu emblematic of this. It was Islam that was associated with modern civilisation and literacy and learning there, not the West. But it was a very different picture further south, where missionaries often came in as or with the first groups to build hospitals, roads, schools and other technological institutions trappings of the modern world.

Islam also had over a millennium to get established in much of North Africa, and most of a millennium in much of West Africa.

That said, conversion to Christianity is far from total, and there are not only major surviving traditional African religions but far more syncretism than is often acknowledged in simple census statistics that just have options like ‘Christian’ and ‘Muslim’ might let on.

This is a weirdly-phrased question, so I'd like to preface with the fact that colonialism is not a one-size-fits-all deal. It's an endlessly complex system of systems and no two places, people, or administrations are exactly the same -- and that's not even factoring in things like time, technology, or outside forces. Even beyond that, I doubt there's any human that can neatly summarize the religious history of one of Earth's larger continents over several centuries in a reddit post.

An answer as broad as your question is "Because some people converted and other people didn't". Why there were more conversions doesn't have a one-size-fits-all answer either, so you have to start asking about specific peoples in specific regions. You have to account for missionary efforts -- when they began, how much support they enjoyed, how they tried to reach people, when they stopped being supported, what their ideas of conversion were, etc. -- as well as the people they were trying to convert -- who listened to missionaries, and why, and who didn't care, and why, etc.

The world is pretty complicated, so here's somewhere to get started: Morocco and the Remaking of French Christianity After the Law of Separation.