If you are European-American (white), your ancestors wrote the rules and kept the records. They were a part of the white power structure that allowed and supported slavery and Jim Crow.
Your records are relatively easy to find.
If you are descended of the African Diaspora, your family is in splinters, and your history is obscured by an official view of you and yours as 3/5 of a human being (check Article 1, Section 2, Clause 3 of the US Constitution).
Records before 1870, for people of color, are frequently “the ungettable get.” Before that, the nation was at war or practicing slavery and was, at best, recording the existence of the enslaved as taxable chattel, identified by gender and age (no names) on Slave Schedules. They had no surnames.
Enslaved persons were also, rarely, listed by name in extant wills that conveyed their ownership between slavers’ family members. Free persons of color, who are listed in Federal Censuses in 1850 and 1860 by first and last name and shown in family groups, were rare.
DNA is a fun extension of a hobby for most white researchers, but for black researchers (and adopted researchers, fostered researchers or researchers with some element of unclear parentage), DNA is ESSENTIAL to building a family tree. Genetic genealogy is not a hobby or “fun” for those in minority American culture, it is compulsory. Majoritarian American populations should remember this as they work.
It’s easy to say “I’m Swedish-American” or “I’m Irish-American” with a degree of certainty supported by documentation of immigrant ancestors’ life activities. Passage records, naturalization papers, and even censuses show white researchers’ national origins, emigration dates, and even cities of European origin.
To be “African-American” is to say “my ancestors were kidnapped from the second-largest continent on the planet, but I don’t know when or where. I have no national or tribal identification. I have no quantifiable or qualifiable roots.” To test DNA in the face of that enormous, gaping hole is to say “science, please help show me who I am.”
For some great information, please read How African Americans Use DNA Testing to Connect With Their Past in The Atlantic, a wonderful digital magazine.
If you are white and you are utilizing DNA testing, please know that existing records are much better for you than for your black cousins. If you are approached by a black cousin on Ancestry, My Heritage or GEDmatch.com, please work with your cousin to identify your common ancestor. Your assistance means the world. So does your education about the real differences between white and black genealogical research strategies.