They chose it because Sig was doing the marketing better than Beretta and Colt, and the price was low.
Even so, you can grab a Beretta M9 for around $550 in just about any reasonable shop, with a package deal for a military unit I am sure it would be slightly lower per gun. Throw in the price of the commemorative engraving and I bet it would be around $650 per gun. I would spend a couple hundred more for a commemorative M9 over a commemorative P250 any day. If the unit decided on the P250 I would just have my own commemorative gun made with more practical gun of my liking. Not only does the P250 have absolutely nothing to do with the military, but I think they are ugly as sin and commemorative engraving on a polymer gun just seems tacky. But hey, it's not my unit or my gun so what I think doesn't matter.