I often wondered myself why rdm didn't get Sanguine blade natively, but the others I already knew it was because rdm wasn't meant to have very efficient prowess. I did notice one thing about Sanguine blade however, that some seem to not notice at all, the WS has a high Enmity generation compared to most elemental WS. Even doing as little as 700 damage with the WS resulted in a easy shift of hate from someone who had established hate already into the fight.

Other than this I can't see why rdm wouldn't have it natively as it complements the build of the job already. ~shrugs~ Then again that is SE for you.