I think a big part of it is pricing to be honest.
Using Audi as a sample I configured the cars as Prestige models with no extras.
A4 All-Road: $53,000
Q5: $50,800
A4: $50,000
You are paying $2,200 more for a smaller car. You get almost 2" more rear leg room on the Q5 and 2 more cf. You also get 2 more inches of shoulder room front and back on the Q5.
As for Europe I checked and attempted to configure as close as possible the UK prices:
Audi A4 All-Road: 45,385
Q5: 43,955
A4 S-Line: 44,015
A4 Avant S-Line: 43,015
It's trickier to configure since engines are different and standard features seem to be all over the place for the UK site. Only options I picked was 2.0 petrol engine and memory seats with virtual cockpit. But the big thing to note is that the Avant is pretty much the same price as a Q5.