Three states (Louisiana, Texas, Washington) do that. Others don't.

As to why, the simple answer is the state government made a law. They get to do that because the state constitution says so, and it doesn't conflict with any federal law. If you keep asking why, you'll usually end up with a root cause like the people who have power want it that way, or at least don't especially want it a different way.

The fact that other places do it differently has nothing to do with anything.