I think that a lot of people would resent, or at least be skeptical of, a professional military man as HS director. Never mind that our police force is made up in large part by past military/reserves.
He'd do an excellent job I'm sure but the distraction may be more than it's worth.
I think the issue is not that he was professional military.
I think it's a personality issue. I don't know why people think, that just because someone is really great in one particular job, for which he is ideally suited, he would be great in a completely different type of job, for which his temperament would be a liability, not an asset.
I've been thinking Richard Armitage .. he's tough, fearless and straight talking.