i think its because he isnt at an official army ranking. He is a state alchemist but he isnt a colonel or a major, so i guess he doesnt have to wear an official uniform. I remember in an episode of FMA brotherhood that there was a state alchemist who didnt wear the uniform, but he wasnt mentioned again because scar killed him.
Not all of the state alchemists wear the uniform I don't think Shou Tucker did and Kimblee didn't wear it for a while too. But most state alchemists do other military work which is why they wear the uniform.
Ed has a civilian status so he doesn't have to wear military garb also he is madami of a field agent and is usually on inspections and travelling, try wearing a uniform for that. Besides who would make such a small uniform. XD Hope I helped.^_^