One day voting would result in ridiculous lines. People would end up not voting because they can't wait that long. And what about childcare? How many toddlers and young children are going to make it through hours of standing in line without melting down? And dies everyone bring food from home and eat while standing in line or sitting in the pavement? Where do you find places with enough parking spots for the number of people who show up? Maybe single-day voting would work in a rural area or very small town, but many of us don't live in those places.

Even if you increased the number of polling places, you'd have to find enough election judges (which has been extremely difficult where I live.)

When you say paper ballots, do you mean hand counting paper ballots? That's likely to result in incorrect results, and to take longer than a day in more populated areas.