White people immigrated from countries with very well-established written histories often going back a millennium or more. Banks kept loan records. Stores kept ledgers. Ships kept manifests. People chose to emigrate and wrote extensive letters back and forth to their family members. Despite coming from all different sects, they tended to be largely Christian. In terms of culture, appearance, and religion, it was easier to integrate into society. With less local backlash, there were fewer major concerted attempts to wipe the trappings of their European backgrounds. All these things make it relatively straightforward to determine family trees, cultural ties, and nationalities of ancestors.

Meanwhile, the African populations that came to America didn't really choose to emigrate. When slavers picked up Africans, the target populations tended to be those that were less organized, less developed, and therefore less reliant on writing systems. When they were taken to the Americas, these populations were often considered to be barbaric heathens. They were forced to adopt English and Christianity, oral traditions were suppressed, and familial ties were obscured. On the auction block, families were split up, and the records that exist did not tend to document information which would establish a family tree. These populations were kept uneducated and disorganized specifically to keep their labor and prevent resistance.

White people had advantages of written records, voluntary immigration, and gradual assimilation. These factors preserved history and allowed them to identify the countries from which their ancestors came. Black people tended to be forcibly removed from societies with less access to writing, kept less educated, their oral traditions were suppressed, and they were forcibly assimilated rapidly. This removed the ability to identify familial ties.

Also, "Ireland" and "Scotland" were established nations for hundreds of years. In Africa, there tended to be fewer established borders and more reliance on ties to tribes. The slave trade broke up tribes and colonial powers rewrote the borders, so tying African history to "nationalities" is less meaningful. France exists now, and still existed in the 1400s. Takrur and the Mossi States existed in the 1400s, and don't anymore. The "nationalities" of Africa lack the continuity present in European nations.

So, basically, white people's nations existed continuously for longer and they had better records to track those ties. African nations have less continuity, and the ability to track modern people to their African ancestors is difficult due to scarce records.