Because there's far greater immersion into shooters with a mouse (absolute precision). Consoles tend to be slow and jerky (clearly obvious to me in the video) with their joysticks.
For example, I instinctually point at the head if there is a head to point to. Games like this (linear with a lot of crap to shoot) literally turn into something like an intricate dance, especially when they are coming from all directions. Ultimately, it becomes a battle with myself to slay the enemy faster, using less ammuniation. That is appealing to me but, I'm not quite sold yet at $40.