IGN- During a session at GDC Europe, Guido Eickmeyer of Deep Silver revealed what he believed to be the secret to Dead Island’s success. It wasn’t the cinematic trailer, though that had an impact. It wasn’t the reviews (Dead Island does not have a stellar Metacritic rating). And it wasn’t the development budget, because Eickmeyer claimed they had nowhere close the amount of money for building and marketing the game that a bigger AAA game publisher would. Instead, he singled out one feature: co-op.
To date, 6500 years of Dead Island have been played cooperatively. This feature, combined with the setting, lots of squishy, shambling enemies and a RPG-like progression structure helped push the game to sell through more than 4 million copies.