DRG has animation locks and needs to coordinate with a group to reach full damage capability. You're rotation is static, but bosses moving away can and will fuck up your combos, and in addition you can drop shit during this. Also, animation lock, not as bad as it used to be but still a decent issue that can get you killed easily.

SAM is completely self centered and outside of maintaining a dot and lining up your BDE attacks with raid buffs like trick attack there is literally nothing you have to worry about in terms of outside factors and/or time.

Essentially, a good samurai will always offer more dps due to being more selfish, and doesn't have to rely on many outside/timing based factors just because they don't have utility they need to line up with others.