Comment by jimrandomh

There's a very important reason to do this which no one has brought up. US elections have a lot of vote-suppression shenanigans; in some noteworthy cases (including the 2000 Presidential election), fraudulently removing voter registrations, understaffing and obstructing poll locations changed the outcome. This sort of thing becomes much more difficult to execute and much more difficult to get away with if there's an expectation of 100% turnout, as in countries which have mandatory voting; large asymmetric chunks of the population failing to reach the polls no longer look plausibly innocent. I think mandatory voting is worth having for that reason alone, in addition to the other reasons.