[srjjs]

Looks like I came a little late, but anyways:

At the uppermost levels, there's little difference between candidates. The average person is definitely able to recognize quality work, but is far less likely to be able to weigh one outstanding choice against another. That's why special committees to grant such awards exist.
That's also why these awards carry so much significance with them. The skill is there whether it's rewarded or not, but the respect that comes with such a presitigious award is extremely important. The recipient, for a large portion of the world, becomes the pinnacle of what that field represents and the role model for many enthusiasts with dreams of someday winning the prize as well.
In addition, the Nobel Prize is something that is typically only given attention to by adults. You'd be hard-pressed to find a teenager that knows who the last few nobel prize winners in any one category were.
(Note that it is not so much the work itself that matters, but what people think of the work.)
These people typically regard Eminem as conveying a message of hate and instability (which, to be fair, he does). By bestowing the highest award in the field on such a person, a significant message is sent. A person with such a negative reputation as Eminem is not a good choice for this award.