The mounting bolt for a front brake is significantly longer than the one for a rear brake. The bolt has to reach across the fork crown which is much wider than the rear brake bridge. You could change the mounting bolt on a rear but that is a fair bit of work and the bolts aren't particularly cheap.

BTW, Nashbar sells their housebrand brakes (made by Tektro) for only $30 for a complete front and rear set. They are decent double pivots and you can't get an individual brake for much less.