The reason that Rap has all the top ten spots is because Rap/Hip-Hop is the only kind of music that actually sells nowadays.
Sorry, that is simply fact.
Another reason, ROCK SUCKS RIGHT NOW. With few exception, most rock cds today are revamped trash. Photcopies of photocopies of photocopies... As Chris Rock had said, "Good Charlotte, more like mediocre Green Day."
He is right, there are no originals anymore. Everyone sounds like everyone else. The last time something new in rock came along was Cobain. (I am not even a fan) Nirvana was an explosion on pop culture. All of a sudden, everyone cut their hair, and forgot how to play guitar, and write complicated songs. It seemed fans wanted rock to boil down to its bare components.
Unfortunately, it has boiled down to nearly nothing. Again, there are some bands that still put out a quality product (Springsteen, U2, Foo Fighters) but for the most part, rock is a blurred caricature of its former self.
Until a band or artist comes along that turns rock on its ear, it will not sell what it used to. Look at concerts. Years ago, any band with a top 40 single was doing stadium tour. Now, Kiss AND Aerosmith can't sell out a 15,000 seat venue.
Another reason Rock is almost dead... the audience grew up. Since the early 90's Rap has been gathering momentum, while rock slowed down. Rap has been a mainstay on radio/charts for years, and as its audience grew out, they also grew up.
I personally hated rap with a passion for over twenty years. It was not until about two years ago that I really began to like it. The reason: Rock just didn't do it for me any longer. It became boring and stale. How many times could I listen to a Van Halen cd from 1978?
I am hoping that rock comes back. I hope that the next big thing is a hard rock band, that writes songs that don't sound like noise, and has a lead singer that actually sings.
Someone mentioned something about why do rap artists "feature" each other... Sales! Why else?
If artist X features artist Y, then fans of artist Y might just check out artist X's cd because it (duh) features artist Y.
Imagine if Jimmy Page worked with Eddie Van Halen, had Freddie Mercury on vocals, and Keith Moon on drums? Sounds like something fans of VH, Zep, Queen and the Who would all check that out.
What is the answer to 99 out of 100 questions? Money.
And just because you don't like something, doesn't necessarily mean it sucks.