Legal Question in Criminal Law in California

It's the federal system where each state is independent when it comes to those type of things.

For the same reason they vary in policies and rules in other areas: Because they each write their own laws according to their own priorities and the interests of their own voters -- and, to varying degrees, of the industries and other interest groups with the most influence in each state.

The federal constitution limits what the states can do in some areas, but those limits still leave the states with broad discretion. Different states use that discretion in different ways.