For an ischemic ulcer, the wound also called an arterial ulcer, the instruction about compression is: Do not wear compression stockings or wraps unless told to by your provider. These may restrict blood flow.
Restricting blood flow is the reason. This page won’t guess at what that would do to a particular wound.
What is clear is why restricted blood flow is the thing to worry about with this wound in particular. Narrowed arteries are the most common cause of ischemic ulcers. This prevents a healthy supply of blood from flowing to the legs, so the tissues do not get enough nutrients and oxygen. The lack of nutrients causes cells to die, and damaged tissue that does not get enough blood flow also tends to heal more slowly. To treat an ischemic ulcer, blood flow to your legs needs to be restored.
So the treatment goal for this wound is more blood arriving, and the warning is about something that may restrict blood flow. The two sit on opposite sides of the same problem. That’s enough to explain why the instruction exists.
The instruction has an exception, and it is a real one: unless told to by your provider. A clinician who has examined the leg and checked the circulation is working from information this page does not have.