First of all, when your question says “the five main reasons,” it sounds as if you might be expected to provide a specific list of reasons. I suggest that you consult your textbook and/or lecture notes to find this list. Different people will have different lists of reasons for this phenomenon. Let us look at some explanations that some people have given.
Some people argue (though this is generally considered to be a racist argument) that people of color are simply more prone to criminality. It is clear that more people of color are arrested, and some argue that this is, at least to some degree, due to biological factors.
A much more common argument is that people of color live in situations that make them more likely to engage in criminal activity. The more conservative version of this argument holds that people of color live in a “culture of poverty.” This culture is said to glorify and encourage criminal activity. The more liberal version of the argument holds that people of color tend to live in poverty and poverty causes people to be more likely to commit crimes.
Other arguments tend to blame racism for the disparity in prison populations. Some people argue that the criminal justice system is racist. They point out things like the fact that people of color are more likely to be arrested for drug offenses even though they are less likely to use drugs than white people are. Other people argue that society as a whole is racist. They argue that our criminal justice system is simply a reflection of our racist society. This sort of argument is much more of a liberal argument.
Thus, there are a variety of reasons that can be given to explain the disparity in the prison population.