Perhaps, like Rebuild, Hunting Party needs cards from its own expansion to hold it back. After all, if the other cards want to you go for a variety-heavy deck, shouldn't Hunting Party be less powerful?
Hunting Party is tough to outrace, but then again there are a lot of really powerful cards in Cornucopia, most of the rest of them work together, and Hunting Party works well with at least some of them (Tournament, Horn of Plenty, etc.).