A reader writes:

Are white men really less present in music? On the Top 40 pop charts, sure. But Country? Nope. Nine of ten there, with the tenth being an all-white mixed-gender group. What about Rock? Nope. Ten for ten there. How about Adult Contemporary? Parity: five of ten for white guys, four white women, and one all-white mixed-gender group.

It seems that what Dr. Science has quantified is not the disappearance of white men from popular music, but the takeover of dance and R&B/hip-hop of the Top 40 radio format – which did indeed occur in the late '80s. Those genres have long been dominated by women and black performers, respectively, so it should be no surprise that when "Top 40" switched from playing music in the then- and still-white-men-dominated rock tradition to playing music from the dance and R&B/hip-hop traditions, we would hear a lot fewer white men on Top 40 radio.

Another writes:

Perhaps white males are not charting in proportion to their numbers because today's music industry is driven less by vocal talent and more by identity and sexuality. 

Hip Hop fans are somewhat obsessed with the artists street cred and it is a genre that does not exactly push the envelope vocally.  You can see the influence of sexuality when you compare the looks of today's top 40 females with those of the pre-MTV 70s.  I love Carole King, but nobody wanted to see her writhing across the stage in her underwear.  So, that leaves room for only so many dorky white guys.

Along those lines:

Since you're only considering chart-topping songs, isn't the real question "Why are white male singers being marketed less?"?

Another reader:

In the US, black has long been constructed as an identity that was essentially opposed to mainstream culture. But we've been undergoing a process in which the nation is becoming increasingly racially inclusive, which includes trends like white people moving back to "inner-cities," increasing exposure to diverse cultural outlooks via the internet, and increasing numbers of Americans claiming mixed-race identities on the census.

The increasing inclusiveness explains why more Americans are interested in listening to diverse types of music. The fact that a "black" identity has historically been constructed in opposition to mainstream culture explains why it's easier for non-white artists to be considered "cool" or "edgy." In the past, the "edginess" of black music was popular, but often filtered through white artists who were more acceptable to mainstream audiences.

Hip-hop is you America!