Because people have jobs, and when people are not working, the last thing the want to do is wait in ridiculously long lines.

On top of this, with mail in ballots you have time and privacy to research all available candidates and proposals. You’re generally not allowed to use an electronic device in a voting booth and researching in line might be illegal in your state if you don’t have a way to hide what’s in your screen; so it’s nicer to actually get informed and vote instead of just the disasterous and genuinely anti democratic idea of “vote blue/red no matter who.”