Why do some countries have common law systems and other have civil law systems?

Answer 1

Just the way their traditions evolved.

Common law is found in England, and the countries that were originally English colonies -- the US, Australia, etc.

Civil law was found in France, among other places, and tends to carry through in places where that influence is still felt.

Answer 2

Common law and civil law are the same thing. Common law evolves from court cases. Statutory law gets made in a legislature.
Both common law and statutory law can apply in a civil case. If a passerby slips and falls on the icy public sidewalk in front of your home, he can make a bodily injury claim against you. That is civil law. Common law would say the homeowner is negligent if he allows a hazardous condition to exist and does not warn anyone. Statutory law would apply if there was a municipal ordinance that said the owner of the abutting property has the duty to clean ice and snow from the public sidewalk.
The other body of law is criminal law.

I could not make any sense out of that first answer.