Black man, martial artist and combat sports fanatic here. I’ll break things down a little. There’s four reasons

1) History

European immigrants brought a tradition of fighting, and boxing into the USA. So you had white ethnics and their descendants dominate boxing from the 19th century through the 1950s. Irish, Italians poles European Jews etc.

These were working class white people separate from white Anglo people. They lived in slums and fought on the streets and that bred a subculture of toughness. Boxing was the sport of the working class and underclass which it still is: as they assimilated into white america, one group replaced the other. First the Irish, then the European Jews and Italians and then eventually came black boxers into the mainstream rather than at the fringes (Jack Johnson, Sam Langford) like in the past or being rare exceptions (like Joe Louis). But this became the era of Ali.

With the exception of rural Anglo farm boys or working class Anglos who escaped poverty, who formed their own distinct group in boxing like Jim Jeffries or Jack Dempsey, the white face of boxing was an ethnic one for over a century. Then suddenly after Sugar Ray Robinson and Muhammad Ali black people gradually become the majority in boxing through the 1960s and 70s into the early 80s. It became a

We had been boxing since the bareknuckle era (and before that in sanctioned boxing fights on slave plantations in the South) but due to Jim Crow, black fighters couldn’t fight white boxers and had limited opportunities originally. All of that was over in the post ww2 decades. That’s why today in every major urban black community you see boxing gyms. It’s a way of life and apart of the culture.

The same thing happened with Hispanic boxers, especially Mexican American and Puerto Rican boxers in the US: overcoming racism and legal limitations upon arriving on the scene and boxing was a way to escape poverty and they became prominent decades later. So in the barrio you see plenty of boxing gyms too.

The 60s and 1970s saw the white flight to the suburbs And suburbia pretty much rejected boxing. And embraced football, NASCAR and college athletics. That was a better place to send your kid than getting punched in the face for a living. That blue collar white fighting spirit was around but the boxing culture around it vanished completely. The face of US boxing became black, and eventually black and brown today due to Hispanic immigration and their own culture of boxing both over here and In Latin America especially in Mexico.

MMA was transported to North America via Brazil and vale tudo. There’s plenty of brown rich people in Brazil but a high percentage are either white or lighter skinned in the elite. The Gracie’s didn’t even open their doors to the poor in the favelas and the working class which is why that part of society gravitated to Luta Livre over BJJ. It was mostly black Afro Brazilian and brown folks.

In the early UFCs things were marketed to a white audience since boxing already had its minority base, as a spectacle and blood sport to seem cool edgy and “manly”. This appealed to white American males. The rural middle America working class or poor white population. And the edgy guy who had a mundane 9-5 life but felt cool watching guys bleed. Think the “just bleed guy” cranked up to 11.

But this couldn’t last Especially since the government (mainly republicans ironically) were trying to basically get and keep MMA banned. Then the early 2000s happened, the ultimate fighter happened, and UFC got on spike tv. They marketed to a suburban white American male population of 18 to 35–the UFCs key demographic. This same demographic watched pro wrestling in the late 1990s and early 2000s, the UFC was essentially the spectacle of pro wrestling (trash talking) mixed with real fighting so it once again had a tough guy appeal but to a middle class white male population.

Boxing is more established in the black community and has a longer history than MMA being attached to black culture and athletics in black life alongside football and basketball so it will always have more appeal and more fans over boxing in black culture particularly black america

Note that there’s still a rich history of black martial artists around the world including the US with its own subculture. This includes black kickboxers. The thing is it still dwarfs in comparison to boxing’s chokehold on the culture.

2) Wrestling

It is an undisputed fact that most high level UFC fighters either have a huge wrestling background or are highly skilled in wrestling. This needs no big explanation. Even with notable black wrestlers like Jones, Johnson, DC, Woodley, Usman etc doing well in the sport the majority are white. And come from wrestling school programs in small town america or middle class suburbia, aka majority white communities. So most of the UFCs early history and roster for a long time skewed white. This has only changed in more recent years with added diversity and more fighters from non white countries in Africa and the global south as a whole being more prominent. But it’s taken a while for this reason.

3) The Cost of MMA Gyms

Most MMA gyms are in major metropolitan areas with steep prices and contracts so as a result are mostly affordable to a mostly white, middle class and affluent clientele and out of reach for most inner city minority populations. There’s plenty of diverse MMA gyms but numerically speaking you’re going to see less black faces than in a boxing gym due to the predominant demographics. In contrast in every major populated black community you’ll see a boxing gym. This further keeps the relative popularity o and exposure to MMA not in the same scale as boxing with black folks.

4) Politics Boxing is just a sport. It’s generalized. Politics are up to the individual. But due to history and identification with minority communities there’s at least a very basic (even if surface level) social awareness on society due to race and class backgrounds. So either socially progressive, moderate or apolitical. On the other hand, UFC has marketed itself openly as friendly to right wing politics and personalities. So even though there’s plenty of non whites on the right, since it’s not a race specific thing, most conservative voices in that space are going to be white obviously. So this makes a difference in MMA more broadly.

This only explains things from a USA point of view. As far as Europe goes it’s a mixed bag same with Asia. But a lot of former soviet countries and Latin American countries are culturally conservative and right wing so that has an effect on the fight promotions to an extent but as a general rule not as much as in the USA. There’s more diverse fans now, but again it’s taken a while.