The simple answer to your question would be that violence and intimidation was used by the KKK members to prevent the African American community from voting.
They attempted to put up monetary taxes in the form of "poll taxes"; which they assumed that the African American population would not be able to afford and was successful to a point. They also placed a "literacy assessment"; which showed that over 60% of the population were illiterate and remember this was also a time when women couldn't vote, as well. So, these African American males were once slaves and were forced into working and not allowed to adequate education allowing them to read. Unfortunately, because of the times, even some white males were illiterate but due to the unjustified racism, the voter clerks would have those men read much easier sentences to pass this bogus "literacy assessment".
The last barrier that the KKK tried to enact would be the "grandfather clause". What this was, was a clause that automatically by-passed the other two regulations (noted above-the tax and literacy), and allowed someone to vote if their actual grandfather was able to vote pre-Civil War; which OBVIOUSLY only catered to the white demographic.
It was unfortunate and it worked for a short amount of time. When this was first enacted it cut the African American voting population from 90% to 6%. However, as the time continued, African American's took their civic duties seriously and they came up for the money for the redundant poll taxes, they learned to read to pass the absurd "literacy assessment" and they were finally able to vote.
Once that stopped working, violence and political intimidation became forefront.
