The main reason why furnaces and cars have it on the incoming side is because the dust and particulate can damage the engine/blower. So you want to filter it out before it reaches there. In a system like this, the box fan will not be damaged by the particulate so you can instead push the air through the filter instead of pull it.

The filters are also designed to have air pulled through them.

There's often an electrostatic component that ties into a filter's efficacy, as well.

The fan controls, and the power cable, stick out of the back, so I didn’t have a choice.