A good reanimator deck can't get a hand full of fatties. It only has like 6 creatures. RB adds more fast mana to be explosive, blue goes slower but more consistent
Lol variance happens bro. In burn you shouldn't draw 3 lands in a row when you only have 18 in the deck, but it happens. A control deck with 20 lands, 10+ tappable sources "shouldn't" get stuck on mana. It "shouldn't" happen, but it does. BR has no way to shuffle away the fatties and if they are disrupted early they really have no way of clawing their way back out, whereas UB does. They are both good decks, it just depends on what you're trying to do.