The US chose this outcome when Eisenhower chose to use federal troops to integrate public schools. The civil rights movement, as much as I personally agree with its ideals, shattered American political unity. It has become politically impossible to make large national investments since the civil rights movement integrated African Americans into mainstream American society.
Whites move out once enough blacks move in, this is a recurring pattern across America. Few white people with the means would choose to send their kids to schools with more than a few black students. Most would not say this out loud, but they vote with their feet and wallets. Allowing African Americans to use public swimming pools and attend the same public schools as white people means that white people will prefer to keep those dollars in their pockets so they can pay for private swimming pools and private schools.
China is also an ethnically diverse country, but the government largely suppresses free speech in favor of nation building. National unity is the national religion in China. Urban Chinese look down on rural Chinese, Shanghainese and Hong Kongers look down all the other Chinese, but the government will never give political voice to these prejudices. The wealth gap within China is as large as between Europe and Africa, if each region took the selfish, short term view the natural democratic outcome would be splitting up the country. Yet over the long run, unity has real benefits. It was because of national unity that China avoided the fate of India and Africa even at its weakest.
What remains to be seen is whether regions of the US start seeing secessionist movements grow stronger and stronger. At that point the US will have to once again choose between democracy and unity.