Why do registered voters end up not voting?

[Stranger in a strange land]

I imagine a decent number of parents make sure their kids register when they turn 18, but don't really try to enforce a mandatory voting policy after that point.

Add to that, a lot of states have "motor voter" laws or programs, so you can register to vote when you get or update your drivers license, after which a lot of people never vote even if they're registered.