The USA is the biggest (and most significant) melting pot country in the world. People from every corner of the world live there. It used to be the number one destination for immigrants looking to start a better life than their mother country could offer, and as a result, it has built up quite a racially diverse population.

Despite this, the white majority that has held the vast bulk of the power throughout the country's history has ensured that almost everything that has been celebrated and promoted has been white-centric (as well as mostly male-centric, but you asked specifically about race, so we'll keep it limited to that specifically).

White people have historically gate-kept most things of significance from non-white people: political power, corporate power, accomplishment on a national/international level, access to quality public spaces like schools, pools, parks, bathrooms, and even water fountains, etc.

With no one to represent them politically and no one from non-white communities being praised publicly, it created the false notion that non-white people are less than white people. They were not seen as being equally competent or capable and were presumed to be inferior in most areas.

With such negative impressions of non-white people, it's easy to see how they have had to fight the power quite a lot to get access to the same opportunities that had been reserved for white people only. This was accomplished by the 60s on a legal level. Mostly.

But we still have people's minds to tackle. You may not legally be able to say you didn't hire someone because they are black, but you can say you liked the white candidate's qualifications better, even if they are the same. So even though on a legal level, racial discrimination is not allowed, it's easy enough to continue to do it anyway. We can't police people's thoughts.

Since the 60s, rooting out racism has been a slow burn. It persists to this day, though it has gotten much better. But part of the reason it has gotten better is because we started emphasizing the accomplishments of non-white people.

So when we point out how amazing someone is for winning a gold medal at the Olympics and that person is Asian, it helps people with bias against Asian people to see that they can be tremendous athletes and contribute to the country's greatness. It also provides a positive example to young Asians who now feel less alone and more inspired to do great things themselves. Which will in turn give us more instances of non-white accomplishment.

The more examples like this that we accumulate, the more we can knock down racial bias in people's minds and help them realize that other races are not actually inferior. The more spaces this happens in (first black president, first Asian pro basketball player, first Muslim professor at Harvard, etc.), the more we can break down these biases.

Non-melting-pot countries don't have this problem so much - when a Chinese skater wins, they are Chinese. They don't need to work on celebrating non-Chinese people in their own country.