This is a very sensitive matter to discuss academically because no religion wants to have scholars saying "here's why this religion exists." Instead, people who believe in a religion tend to believe that it is true and that is why it exists (while all other religions are in some way false).
Scholars would say, though, that there are different religions because people in different areas of the world experience different conditions and have different cultures. Their religions, therefore, come to reflect this.
For example, all of the world history books I have taught from say that ancient Mesopotamia had a religion whose gods were much harsher and scarier than those of the Egyptians. They say that this was so because Mesopotamia had harsher conditions -- life was harder and so people believed in a religion with harsher gods. The MNSU.edu link says
Today, there are numerous challenges and problems faced by humans from every possible background, location and social class. Every day people must face issues of health, safety and mortality. It is because of these daily challenges that religion continues to exist. Religion is the universal tool for explaining things which we do not understand through the context the known physical world.
This sort of explanation assumes that religion comes from the needs of the people. Because people in different places and situation have different needs, different religions arise.