I think it tends to be based on how they recruit players. VC is a bookmaker in the UK and therefore gets a lot of people that are gamblers by nature and don't really know a lot about poker. The same goes for Pacific as they recruit via their online casino 888.com. I believe
bodog is a sportsbook and a casino as well.
I think the sheer volume of players at party with it being the best known mean they get a big variety of players and therefore the good to bad player ratio is probably heavily in favour of bad players, similar to Stars I guess.
Never played Paradise so I can't comment on them.