Exterminatus is being developed by much the same people as worked on the Rival Species mod for Half-Life 1. Source was a logical choice as it has much the same structure for modding, reasonable documentation and tools familiar to HL modders as well as being a graphical upgrade, incorporating physics and displacement terrain.
Well, okay, but you could have gotten better graphics and physics if you would have upgraded to, hell, not even the Unreal 3 engine, just the Unreal 2.5 engine. "It's easy" doesn't really churn out as a good excuse to me, the potential player, when it means your mod was less than it could have been.
Yeah and it makes it accessible to loads more people since, and this is taken from statistics, the games on the source engine are played by many, many people.
Who doesn't have HL2DM? Or Gmod?
Wait, they're aiming for profit?