I think up until recently, in most countries the law was such that you could only purchase duty free items when departing.
The other issue is that departing passengers usually have more time to spend (or waste), which explains why the duty free shops in the departure areas are so prolific and those (where available) in the arrivals area are not. Particularly when faced with the prospet of queues at immigration, most pax just want to get off the plane and through immigration presto.