This is simply not true. As mentioned, Tim Scott has won two statewide elections in South Carolina (a special election for the Senate seat and a general election). The first post-reconstruction African-American Governor in America, Douglas Wilder, was elected in Virginia in 1990. In the 2000s, Michael Williams was elected to the Texas Railroad Commission, a statewide office, several times and Thurbert Baker was elected Attorney General of Georgia three times. I’m not saying it’s common, but it happens.