The mechanisms of evolution have also been attributed to the development of different religions. One guess to the reason why there are so few religions with no obvious reward system is that it's more difficult to convert people to these, compared to major religions like Christianity and Islam where there are great, clearly defined rewards and punishments.
I guess, at one point in time, there were a lot more religions with neither reward or punishment system in place or just either one of these, and adopting more feedback systems to a religion at the time greatly boosted its breadth and penetration across populations.
I don't remember where I heard this, but I wouldn't be surprised if it was true.
Maybe they should try harder then.