A few comments about voter turn-out in the US...
1. For a large portion of the country, voting really doesn't matter. If you reside in a partisan district (heavily GOP or Demo), even if you are moderate/independent, your single vote might not matter. Since the early 2000s, the GOP has controlled many state governments and redrawn voting districts heavily in their favor. Some of these are being challenged right now (and several have been deemed unlawful by various courts), but it will take many election cycles to undo that damage.
2. In Presidential elections, #1 holds true. But, even worse, due to the electoral college, some states have outsized influence on the outcome. Voters in CA have less voting power than voters in North Dakota. This is due to each state having a minimum number of electors, but a fixed total number.
3. Even ignoring #1 and #2, at the national level, there aren't big differences between the GOP and Dem positions on the things that will impact an average person's day to day life. Elections always come down to "those guys kill babies" or "those guys like to arrest black people". This is less true at the local level, where property taxes, schools, etc actually do impact daily life, but for whatever reason, those elections are less interesting to most people.
4. Voting in the US is during work hours; it is not a holiday. Hourly wage-earners are frequently forced to forgo income to vote. The GOP deliberately understaffs polling places where possible to increase wait times and make voting as tedious as possible. They also regularly attempt to stymy voting through ID programs, misinformation campaigns, intimidation, etc.
5. 24-7 news cycle has many people worn out and frustrated with the system. There is a growing sense of "fuck it, let the whole thing burn" on both sides of the aisle.