Because it inspires a sense of shock and awe, and can leave burns on a target which are hard to heal as opposed to a simple stabbing injury from something like ice.

I guess lighting is an even better choice, since it's cheap and tends to seek things, and it can leave burns too and screw up someone's fine motor control.

[SURELY IT IS NOT SUSPICIOUS THAT ED HAS THOUGHT OUT COMBAT APPLICATIONS FOR MAGIC THIS MUCH.]