Post-match rewards are random, they aren't based on how well you do. Why? Well it's simple to understand, if the people who got 25+ kills a game (or lots of caps/defends) got all the good loot, people would stop playing Crucible. It's supposed to be "fair" in the sense that you can do good or bad and still have the same chance of getting loot. I don't agree with it really, but I usually tend to get some sort of reward per match, rarely do I go a few games without getting anything.