Because a still photo doesn't show the velocity of the pass, how much Cooper had to shift his body mid-route to make the catch. You, and most people here, understand how much more difficult it is to catch a pass when your legs are going full speed in one direction and you need to twist your body on a freakin' dime to catch a ball behind you. Was it "catchable", a lot of difficult passes are catchable, it doesn't change the fact that it's a difficult catch.
Let's just focus on this though - EVEN IF Cooper dropped the ball and it hit him in stride, that doesn't end up an interception. It drops to the ground, because it was behind Cooper, it ended up an INT. Have some of you just gone insane with the fan club? What is happening here? This shouldn't be as debatable as some here are making it. It's the same thing if you throw a ball a little too high, even if it's "catchable", it still runs the risk of being tipped and then it becomes a drill to who can get under it.
This whole "if you get two hands on it, you should catch it" - it's the same thing for QBs needed to hit a WR in stride. This goes for every other route, you wouldn't excuse a QB who underthrows a WR that has a step or two on a corner on a post route, would you? Hit the freakin' receiver in stride.