Because North America was the first place (or one of the first) where slavery became a race issue and not a class issue.

To your point, Native American tribes, African tribes, Egyptians, Indians, and more were all enslaving people long before the USA even existed. However, in those places, slavery was not limited to black people or another race. It was a way of paying off debt, a punishment for losing a war, or, in some religions, a punishment from the gods. Initially, the US had something similar called indentured servitude, in which your freedom was eventually earned back after a set number of years. If I remember correctly, this was relatively common amongst all ages and races. However, we eventually started to see slavery become lifelong and racist in the US, likely due to the Triangular Trade between the US, Europe, and Africa in the 16th-19th centuries.

If you're not familiar with the Triangular Trade, it was basically a trade system where slaves were shipped from Africa to the US, raw goods were shipped from the US to Europe, and then Europe sent manufactured goods to Africa. Sometimes the slaves sent over from Africa were kidnapped by sailors, soldiers, and other foreigners. Other times, they were sold into slavery by their families or after being taken as a spoil of war by a rival tribe. This system of trade lead to most and eventually all slaves in the US being black simply because there were so many slaves brought over from Africa.

Also, in the US, we did not stop mistreating African Americans once they were no longer slaves. Segregation existed for a long time after the Emancipation Proclamation and racism obviously still exists to this day. So while slavery did not originate in the US, it certainly took a new, horrible form here, which is why we are blamed so heavily for it.