Mostly the reason is to keep the reward in line with everything else in the hard roulette so they don't have people leaving duty because they got one of the lower rewarding dungeons. Frankly I agree that they are just a little too stingy with soldiery as it is. But presumably the new expert tier dungeons will be back up to 45. And hopefully with hunts, frontlines and syrcus tower soldiery won't be quite so obnoxious.