Obviously there was discrimination and Jim Crow with a sizable black population, most of whom didn't vote or couldn't vote.
This is most of the answer right here. Remember, some of your data is before the Great Migration and before that the vast majority of black people lived in the South. So, most of the black population was subject to legal discrimination and a campaign of terrorism.
There's a few things, most of which also fall under the main heading, Jim Crow but may have affected others.
1. Southern states had poll taxes. So, you had to pay a fee to register to vote and each year when you voted. These were low in dollar value, but the South was poor and most people were farmers. Tenant farmers had most of their assets tied up their farm, most of the year. So, that prevented a lot of poor whites from voting to (if the laws were actually applied to poor whites).
2. Literacy tests. A higher percentage of the population was illiterate, especially in the South so this had an effect.
3. Democratic Party domination of the process. The South was very corrupt and there was a lot of ballot box stuffing and votes were thrown away. The primary process was even worse. Some states actually banned blacks from party primaries under the idea that a party was a private association. So, by the time the general election came around, these Southern states already had a horrible race-baiting segregationist as the only viable candidate.
