Because of poll taxes and voting machines

Poll taxes were not about charging a large amount of money to be able to vote. It was about figuring out how and to whom you pay the tax. You have to hunt down that small hidden office that will give you a reciept so you can show it at the polling location. That is the exact same affect all these voter ID laws have. You have to find the right place to get the correct ID. You have to do it far enough in advance to meet the arbitrary deadlines. And you have to carefully research to make sure you get the correct one.

Then there are all the little failures in the machines designed to make a skeptic out of intelligent voters. The machines are so full of problems and cheating opportunities that it makes you wonder if it is even worth the effort to vote.

Then there are election day last minute changes like closing down or moving voting locations and fellon lists that are merely methods of getting liberals not to vote. Then there is gerrymandering and caging lists.

Then there is crossover voting designed to prevent valid primaries.

I could go on. None of these scams by themselves would significantly reduce the number of voters. But taken as a whole they significantly impact the number of voters who are poor.

The only way to overcome them is for massive turnout of a motivated electorate. And that is difficult to orchestrate unless you have the possibility of an historical outcome. Like the 1st Black president or the 1st woman president.