Can somebody please explain 1950d-60s South Carolina to me

This is 1952, as you can see, Ike wins the Lowcountry counties while Stevenson wins the northern ones

This is 1956, Unpledged dixiecrats win in the Lowcountry while Stevenson again wins in the northern counties

1960 and 1964 sees republicans again win the lowcountry counties while the democrats win the northern ones, Goldwater in 64 winning the state because he could break into the northern counties

Yet its only in 1968 that the northern counties vote for dixiecrat Wallace while the lowcountry go for Humphrey??

Like I get that only in 1968 could black people actually vote much, but why did the northern counties only go for the dixiecrats in 68? Why did they not break party sooner? What caused this?