Missionaries have been in India for centuries but Hinduism is still the dominant religion, in China its still comparatively small, Native Americans fought against it for centuries. But in a countries like Malawi or Zambia, which were only discovered late on, Christianity became the dominant religion relatively quickly.
A few reasons:
Africa was much more divided than china or India. In china you could see Confucianism deeply rooted and in india hinduism was deeply rooted in society as well. This acted as a Unifying force that helped them repulse the christian missionaries. But in Africa this was entirely the opposite. You could see their religion change drastically from village to village. This made it a lot easier for the missionaries to convert a small village than a large tribe. It is much easier to conquer a divided population than a unified one.
Another reason is that china was relatively free from european influence until the 19th century. They were extremely isolationist for a long time and limited European presence to a few costal cities. But the slave trade in Africa made Africa weak and much easier to conquer since they had no modern government and their population was lower than that of Asia.
Some Europeans also saw it as their duty to help develop poorer or less civilized areas of the world. The White Man's Burden by Rudyard Kipling is a great example of this. Since most of Africa didn't have a efficient government or no government at all they saw that they needed to be civilized faster and on need of it instead of Asia where they had Semi-effective governments at the time.
There is many more reasons, and I'll add them if i remember them.
Sources: AP European History course. A History of Western Society sixth Edition (McKay et al)