We're not talking Vogue here...

The models are there to serve a purpose and they do. I've noted over the years a few come and a few go. Some of the current crop of newer models brought in within the last few years don't really strike me as anything special and I personally preferred some of those who have left or whose air time is minimal, though I like some of the longer term models who have remained.
I'd guess QVC likes the continuity that having a stable, well, stable of models provides. Besides, I think they assume, probably rightly so, that some viewers have bonded with certain of the models...
People complain that they want more older models, bigger models, models in wheel chairs and so on, but at the end of the day, personally, the models do what needs to be done and simply aren't that important to me. Besides, QVC can only have so much of a mix and only have so many of 'the girls' on contract, and somewhat justifiably so... Their mix of models seems pretty reasonable.
In my pantry with my cupcakes...