Well, I am also having trouble with stating it as a fact, that the reason was they were too wealthy. Might have played a role later, but that is not clear to me from what is stated on wiki:

"The massacre began during Memorial Day weekend after 19-year-old Dick Rowland, a black shoeshiner, was accused of assaulting Sarah Page, a white 21-year-old elevator operator in the nearby Drexel Building.[25] He was arrested and rumors spread that he was to be lynched. Several hundred white residents assembled outside the courthouse, appearing to have the makings of a lynch mob. A group of approximately 50–60 black men, armed with rifles and shotguns, arrived at the jail to support the sheriff and his deputies in defending Rowland from the mob. Having seen the armed black men, some of the whites who had been at the courthouse went home for their own guns. There are conflicting reports about the exact time and nature of the incident, or incidents, that immediately precipitated the massacre.

According to the 2001 Commission, "As the black men were leaving, a white man attempted to disarm a tall, African American World War I veteran. A struggle ensued, and a shot rang out." Then, according to the sheriff, "all hell broke loose."[26] The two groups shot at each other until midnight when the group of black men was greatly outnumbered and forced to retreat to Greenwood."