Why is it almost impossible to learn a mathematical concept on Wikipedia? They are very difficult to follow, especially if one doesn't have a solid background in the subject.

One of the many things that Wikipedia is not, is a textbook. There is a reason why textbooks are written one way, and encyclopedias are written in a different way.

As others have said, it is possible to learn a lot of mathematics from Wikipedia, but by design it is not structured in a step-by-step sequence to facilitate learning from the ground up. Nor should it be.