Why? Because the legitimacy of an election depends on the electorate accepting that it was fair, that everyone who tried to vote got to vote and that every vote counted. Lose that, and your voting system might as well have suffered a devastating technological attack.
Unfortunately, in much of the United States, we are no longer able to assure people that none of those things has happened. A recent poll shows that 46 percent of the American electorate do not think their votes will be counted fairly, and about a third think it is likely that a foreign country will tamper with the results.
That loss of trust has itself become a form of voter discouragement. Why vote, when you feel it may not matter? Why register, when you fear you may be tossed off the rolls?