I assumed it was because the shops already had stock of that model, and had paid the manufacturer for them based on being able to sell them as RRP or near enough.
So if they don’t sell quickly, they pile up on the manufacturer’s shelves, and they can’t force shops to discount them as the shops don’t want to lose money. So the easiest way round that is to just offer cashback to get stock moving. Then once the deal’s over reduce the trade price to the shops for new stock.
That or they just made claiming enough of a faff that enough people never bothered. I’ve had phone deals where the cashback had to be claimed in 3 cheques, by sending in paperwork between certain dates over 2 years.