Maybe Eminem feels like there isn't a rapper good enough to be on his album. Maybe to Eminem, in order to make an indefinite classic, and a sequel to his best album, and quite possibly the best rap album ever (imo), he has to only include the best of the best. Maybe Eminem doesn't feel that anyone is good enough for him anymore. Kendrick, in all honesty is the only person on par with Eminem, that is still making music (maybe Nas), and Kendrick has only just begun.
Look at the rap features on the first Marshall Mathers CD:
RBX
Sticky Fingaz
Bizarre
Dr. Dre
Snoop
Xzibit
Nate Dogg
D12
Are any of them relevant now? Do any of them even have fans anymore? The exception is Dre, and X (barely X). But Em would write Dre's verse, and X and Em had a falling out apparently.
I'm just throwing it out there, maybe no one is good enough for Em anymore.