why did the human and chimpanzee lines split?
This is actually not a simple question, because it requires that you have some knowledge of how evolution works in the first place.
There are many mechanisms by which an ancestral lineage of organisms can split into two (or more?) new descendant lineages. These are called mechanisms of speciation. First, you need to consider how species can be separated from each other. (The biological definition of a species is "a group of similar organisms that can interbreed in nature to produce fertile, viable offspring." There are other definitions, but this is probably the most commonly cited.)
It's also important to understand how evolution can happen. You can read an overview of the basic mechanisms here:
Finally, you need to know the various ways that one species can evolve into two species:
Once you read and understand these mechanisms, you will see that the question of the evolution of humans and chimpanzees from a common ancestor is more of a "how" question than a "why" question.
As to "why" that ancestral lineage split: it's anyone's guess. Many of the mechanisms above may have been involved, and we cannot go back in time to reconstruct the entire process. But understanding the process gives us an idea of HOW this could have happened, even if we never know why.
I hope this helps.