Plasma donations:
- take longer (45-60 minutes vs 15 or so for whole blood), so fewer people make this time "commitment"
- are particularly useful for burn victims
- the quantity of plasma drawn would take 8 - 10 pints of blood for equivalent volume.
I think the last bullet is the main reason some places pay for plasma.
I've donated plasma for years and never been paid. I wouldn't take the money - though I have taken a couple of extra packs of cookies for the kiddies.

Wumpus....