The invention of the cheap printing press and good portrait art undermined the historic brutality.
As Muyart wrote in the 1700s, several factors started to make public executions (which gladiator fights were essentially) less popular.
The rise of novels made the general populace identify more with strangers. Previously, much of empathy was for people in your tribal group, and the invention of the novel encouraged people to sympathize a lot more with strangers. Reading fiction encourages empathy a lot more than many other portrayals, and makes especially brutal displays less popular.
Solid portrait art also became a lot more popular, along with the idea that we should view people as individuals, rather than representatives of groups.
Together the two ideas, even outside of Christianity, made people less reluctant to see bloody executions. When people watched executions, more and more there was a sad silence, rather than joy and jeering. For example, this visitor to the 1787 France execution of a man by breaking.
The noise of the multitude was like the hoarse murmur caused by the waves of the sea breaking along a rocky shore: For a moment it subsided, and in an awful silence the multitude beheld the executioner take up an iron bar, and begin the tragedy, by striking his victim on the fore arm.
Of course, there was an exception to this empathy that survived to the 1950s- black people. There were regularly vicious and brutal fights arranged by black people, under slavery and after with financial motivation.
One simple change you could make to make gladiator arenas more common is have the confederacy win the war. Then slavery of black people remains common, and brutal displays with black people can remain common.
The confederate America can spread their ideas throughout the world, and encourage people of races they dislike to fight to the death for their amusement.
Later on, seeing the fame that black men would gain from the arena, white men would no doubt demand to fight as well, and all could have race wars in the arena for the pleasure of the American empire.