Expending blood and lots of treasure to conquer Mesopotamia and immediately lose it due to one's own stupidity and failure to fully think things through isn't usually considered to be a Good Thing either. I dunno, I'm not particularly well versed in history, but something like that could hurt your career.Eh, might seem arbitary but dickering around on the edge is rather a better thing than being dickered around inside the country. I dunno, the Civil Wars hold far more terror than beating up on Dacia?
As for Dacia? Eh. I've never seen the point of the Dacian campaigns except as a salve for Traianus' fragile little ego. At least with civil wars, one usually doesn't have the element of choice; it's hard to see how Constantinus, for instance, could have avoided fighting with Maxentius. Traianus absolutely could have avoided his Dacian wars.