@SWA and @Amadeo make excellent points.
Don't think of it as Em releasing the best material in his vault, think of it as him releasing the material that best describes the particular phase he's intent on showing the world.
Relapse was brilliant, twisted and out of touch with reality, like a distorted image in one of them weird ass mirrors. A perfect, I would even say genius, representation of Eminem's mind state or the mind state of any great artist creating in isolation; dis-proportionate, unbalanced, obsessively focused on detail but neglecting the whole.
In other words, the sort of thing Kanye would get a perfect score on Pitchfork for.(Sorry, couldn't resist)
Recovery, OTOH, was loud, bombastic, I'm-trying-to-convince-you-as-much-as-I'm-trying-to-convince-myself music; anthemic, triumphant with more than a tinge of desperation, and as with Relapse, waaaay more complex than it is given credit for.
So yeah, TL;DR, Em is much more of a showman than anybody seems to realise, giving glimpses into his personal life, teasing but not quite revealing the entire package(PAUSE), that is what makes him so interesting, why he can do 800k with little promo, and why he seems to be getting even bigger(also PAUSE) when he's supposedly past his prime.
Relapse, love it or hate it, is like a time capsule, so is Recovery, capturing a particular moment, giving little glimpses into the myth th2at is Marshall Mathers, and if it leaves you dissatisfied and wanting more, well, THAT WAS THE WHOLE FUCKING POINT.