People would prefer sniffing less powder to achieve the same high.
(4) 20mg oxycontins breaks into a lot more powder than (1) 80mg pill. So for people that sniff and/or IV oxycontin, the 80's make it a lot easier.
For sniffers, the membranes in the nasal cavity can only come in contact and absorb so much powder. So less is more, since a smaller amount of powder can potentially come in contact and be fully absorbed by said membranes.
For those that IV, they are looking for a rush, and it is a lot easier to dissolve (1) 80 than (4) 20's. So they can get an 80 in one shot, whereas it may take more than one shot if they are dissolving all of those 20's.
We aren't allowed to discuss price here, but I will say that the opposite seems to be the case by me in terms of which way is cheaper to get the total of 80mgs. Kinda like getting deals on buying more than 1 type deal.