Defsounds.com wrote:
The Three Headed Monster has cleaned up Billboards decade ending charts. Eminem, 50 Cent and Dr Dre all take top spots.
Eminem ends the decade on a high note, as he was named Billboards Artist Of The Decade. In total, Eminem has had 28 track entries on the Hot 100 chart since 1999. Em’s top hits on the chart include My Name Is and Lose Yourself.
Billboards Top 10 Artists Of The Decade are as follows.
1. Eminem
2. Usher
3. Nelly
4. Beyonce
5. Alicia Keys
6. 50 Cent
7. Nickleback
8. Britney Spears
9. Destiny’s Child
10. Jay-Z
50 Cent can join Eminem on top, as he was ranked the Best Rap Artist Of The Decade. 50’s award stems from his hit single In Da Club. 50’s single was Billboard magazine's number one single of 2003. It was the first time since 1994 that an artist had both the number one song (In Da Club) and album for the year (Get Rich or Die Tryin’). 50 spent 45 weeks on top of the Rap Songs Chart this decade, more than any other rapper can claim.
Billboards Top 10 Rap Artists Of The Decade are as follows
1. 50 Cent
2. T.I.
3. Ludacris
4. Jay-Z
5. Nelly
6. Kanye West
7. Lil Wayne
8. Missy Elliot
9. Ja Rule
10. Sean Paul
Now we have Eminem and 50 Cent leading the way in their respective categories, and we all know you can't have Em and 50 without Dr Dre, so we have the good Doctor on top of the Decades Best R&B & Rap Albums chart with 2001 also refered to as The Chronic 2001 by some. 2001 was the second studio album released by Dre and it debuted at number 2 on the Billboard 200 charts with sales of over 550,000 units. Within the 1st year of its release 2001 was certified six times platinum in sales by RIAA. And we cant forget it also helped to establish Eminem’s career with the hit single Forgot About Dre.
Billboards Top 10 Hip Hop/R&B Albums are as follows
1 Dr Dre – 2001
2. Eminem – The Marshall Mathers LP
3. DMX – And Then There Was X
4. Nelly – Country Grammar
5. R. Kelly – TP-2.Com
6. Mariah Carey - The Emancipation Of Mimi
7. Eminem – The Eminem Show
8. Usher – Confessions
9. 50 Cent – Get Rich Or Die Tryin
10. 50 Cent – The Massacre
Snoop also ranked on top of the decade list for Best Rap Single with his hit Drop It Like It’s Hot featuring Pharrell.
The Billboard decade ending ratings are compiled from information gathered from dates December 4th, 1999 to November 28th, 2009.
For a complete list of Billboards Decade Ending Charts click http://www.billboard.com/charts-decade-end
Source: http://www.defsounds.com/articles/The_t ... the_decade
I guess Eminem has created his own genre then 
Can't see any other explantion to him being top artist and not rapper.