Their offers for existing customers are designed to promote buying expensive items in a short period of time. A pretty shameless milking of the cash cow (or treating their loyal customers as fools, if you prefer).
They are too arrogant to offer what would get a higher volume of customers ordering at a lower value - I reckon if they sent a free delivery code to their existing customers it would be well received. Of course with how trusting QVC typically are of their customers, and how inept they are generally I assume they expect the code to be all over social media and used by every tom, dick and harry (which wouldn't happen if the code could only be used in conjunction with the account customer it was sent to, but I think that's beyond their capabilities). If they genuinely negotiated such fabulous discounts from all the brands they work with, offering free delivery wouldn't faze them at all... but truth is closer to: they use the delivery charges as their universal panacea - it covers the lack of discount in some cases, pays for all the "free" phone lines, and delivers the profit for their head office. No chance QVC will put their hands even slightly in their own pockets for all the £1,000s spent by their customers already - no, they want to squeeze them until their pips squeak.