The Ku Klux Klan realized that it would be harder to keep African Americans down if they were economically successful because they would be able to influence things more with money on their side. They therefore targeted them so that African Americans would remain poor and easier to oppress.
Other reasons included:
They would employ other African Americans - Economically successful African Americans might employ others as well which would make more African Americans successful which would reduce the number of Blacks that could be discriminated against.
To kill inspiration - Discrimination against people works better when they feel they have no hope of getting out of their predicament. Economically successful African Americans showed that there was still hope so in attacking them, the Ku Klux Klan wanted African Americans to remain hopeless and disenfranchised.
