Why was St. Edmund made a saint?

Because was a martyr.

The Danes captured Edmund alive and attemped to forced him to give up his Christian beliefs. Their intent was force his conversion to their religion, and then turn his kingdom into a vassal state of their own. Edmund refused, and was then shot to death by Danish arrows for his beliefs.

Since he clearly died for the Christian religion, he is a martyr and was then canonized a saint.