It's because of burst AoE. Keep in mind that a pull typically lasts only a few GCDs (you won't focus on Bone Shield, while you might Marrowrend 1-2 times to have one up, you definitely won't focus on keeping stacks above 5), and nearly all of your damage is going to come from Blood Boil and Bonestorm over a very short period of time. Crit is the primary stat that will affect your damage here - the benefits of haste only show up on fights of a normal length (which is what I assume you simmed for).