Well when the voting system was first designed, they wanted to essentially make it so only white male landowners could vote, because many felt that those would be the only ones who had the time to follow politics to know what was going on, hence why they made the day of voting in November on a Tuesday for only a few hours. Knowing that most people wouldn't have time to leave work to hit the polls. This allowed them to effectively exclude people from voting (as they could potentially throw things off) without outright saying it and looking bad. Now there are isolated incidents where they allowed women/minorities to vote, but it was quite rare.
Fast forward to today, our political system is long and brutal, and filled with attacks rather than ideas. The idea behind this is to disgust voters so badly that they wont participate. Both parties know they have their own hardcore voters who are going to vote for them based on the letter next to the name and virtually nothing else, so in order to keep unknown voters (that may vote for you, but may also vote for the other guy) away from the polls is to make the political season so long that people are incredibly sick of it by the time it's actually voting day. That's why we drag out the election season years where most countries do the same thing in months. They also run so many attack ads hoping to disgust the other sides voters so badly that a few of them stay home, and thus they get the edge. Less voters hitting the polls, easier their job is. They spend millions of dollars to ensure that the system remains long and painful, when they know they could do the same process in a month, spend a fraction of the cost, and making voting over a weekend/a three day holiday, and see voter turnout skyrocket. That however leads to unknowns, and politicians hate unknowns. The US knows they cannot outright deny people to vote based on race, gender, social status etc, so the next best thing is to make everyone so pissed off that they stay home.
That's also related to why they play to the lowest common denominator. Now they want more stupid people voting (not saying that voting is stupid) because they're less likely to do the proper research, allowing most of the politicians to remain in power rather than get voted out for what they do (or don't) do. That's why we can have a congress with lower approval ratings than dog shit, cockroaches, traffic jams, hemorrhoids, toenail fungus (any many many awful things) and yet continue to get reelected consistently at a better than 90% rate.
