u/Hillsonghoods answer/links should be very helpful (As someone who is interested in the history of popular music themselves, especially in the U.S., he's probably one of my favorite approved historians here).

A large reason for this is that, a lot of popular music today is rooted in African American and Afro-Caribbean musical roots and trends. Rock/rock n roll has a large part of it's roots in 1930s-1940s "rhythm & blues" (Which was basically just pre-1950s rock n roll that was predominantly being spear-headed by African American musicians), which has it's roots in earlier "country blues", but also contemporary (At the time) jazz trends.

The same can be said for reggae and ska. As well as drawing on musical trends in America (Such as rock and RnB), reggae and ska have deep roots in previous Afro-Caribbean music, such as calypso and mento.

Most of this music is a broad, complex mix of west African musical ideas that were kept in the memories of, and reinvented in the new world by enslaved Africans, that in itself was mixed with many western European musical ideas that would have dominated the Americas prior to about the 19th century.

Speaking of the 19th century, the influence Afro-Americans (Both those of the Caribbean and North America) have had on American music goes back much further than the 20th century. One large example of this is the banjo. This instrument, in large part, developed from gourd-body lute instruments from west Africa. These (Or perhaps the idea) of these instruments were transferred to the Americas through the Atlantic slave trade.

Slaves in North America and the Caribbean began to develop these instruments out of what they could find on plantations (One of the earliest portrayals of a banjo-like instrument in America is in this 18th century painting titled 'The Old Plantation', portraying a makeshift gourd instrument with a western-style finger board and short drone string, characteristic of the modern banjo) and in early America, this instrument became culturally solidified as a "black/black slave instrument".

Ultimately, by the 1830s, the banjo became a large part of the extremely racist but also very popular at the time blackface minstrelsy caricature. Many blackface performers not only adopted the banjo itself, but also the unique playing style ("clawhammer") and the rhythms that come from it which was also likely brought over from west Africa. Upon adopting elements of early black American music (And often exaggerating it for humorous effect) popular music in the Americas had already become inadvertently but strongly influenced by Afro-American music.

While the roots of blues is something that is not as clear cut, a lot of the genres distinct aspects can probably be safely traced back to parts of west Africa that has had strong Arabic/Islamic influence on it's residing cultures, as well as other west African music to a lesser extent. To get an example of this, I'd check out this album of music, recorded by Samuel Charters, which records numerous types of music from along the coast of west Africa (Primarily in and around Senegal and the Gambia) and attempts to compare such music to the blues, as well as this video, apart of a collection of field recordings by Nana Kimati Dinizulu, which attempts to draw similarities between west African music (Mostly Ghanaian) and the blues. It should be noted, while similarities can be drawn between blues music (And Afro-American music in general) and the music in these recordings, many differences can also be drawn.

All in all, the short of it is, "black people", or more accurately "Afro-Americans" are so prominent when it comes to music because the world, by and large, has been strongly interested in their music, which is somewhat unique to them, developing (Like many parts of their culture) out of their complex history in the Americas as people descended from slaves who were originally from west Africa.

You may enjoy previous answers of mine on similar topics such as:

• African-Americans invented rock, jazz, blues, and rap. How did one marginalized minority have such a large impact on music?,

• What is the origin of black American music? i.e. the blues, ragtime, etc. I understand that slaves brought their music with them from Africa, but can we be more specific? Which African countries had blue notes? or the distinctive rhythms of ragtime?

• What are the european american cultural contributions to blues, jazz, rock and roll etc? Ive read a lot on how African Americans blended their traditions with European traditions, but it never goes into depth usually just stops at "melody"

• Why are there so few famous black rock musicians after Jimi Hendrix? Had African American pop culture already moved past rock to funk and Motown?

and

• From Ska to Rocksteady and Reggae to Dub, Jamaican music w​as transformed ​at an incredible pace in the 1960s and '70s. What factors influenced these ​major ​artistic and technological advances?

Hope that helps!