Because QVC is a business - businesses (should) set prices that will cover their operating costs plus make a profit for shareholders and reinvesting.
Their current model covers their costs and returns a profit acceptable to them. If they didn't charge for postage, that loss would eat into their overall profit margin - unlikely to be sustainable without recouping it in another way. As they are a retailer, that is most likely to mean price increases.
Anyway, at the moment they have no incentive to change things - they are profitable, customers continue to buy, so (from their perspective) why should they?