Because he's only half Gem. Gems can regenerate from fatal injury, but humans can't. While it's never been made clear what exactly Steven is, the most plausible theory I've found is that he's a clone of Greg formed on a hard light matrix. While genetically pure human, he has the abilities of a Gem. Going by this theory, when Greg's cells eventually die, be it from age or injury, all that will remain is Rose's gem. Since Steven's memories and personality come from his human brain, only Rose's memories and personality will be left in the gem. Like I said, it's just a theory.
I forgot how awesome that song was until I listened to it a few days ago. Been a fan of the band since childhood.
XD True; no mortal man could be involved in QOTSA, Them Crooked Vultures, Kyuss and Eagles of Death Metal, that is too much awesome for anyone to handle