Look at how the lower container sits in the train car, the door is blocked by the car. The top one isn't blocked by anything and they'd just have to cut the locks. The doors on those are just like the back door on a semi trailer (we have a conex that we use as a storage shed).
Also, they're looking for valuables they can resell. I don't know how willing people are to buy covid tests from some guy who stole them off a train.