Why Is Anesthesia Either Epidural Space Or Subarachnoid Space, But Not Subdural Space?

The subdural space is only a potential space, not a continuous space suitable for puncture. The dura and arachnoid membranes are almost stuck together. The short answer is that you can only find this gap in the anatomy, but it's very difficult to penetrate it.

When you have an epidural, there's a very small chance of inserting a tube into the subdural space and having a subdural anesthesia, and that can happen.

In addition, your statement is wrong. The main method of anesthesia is general anesthesia.