Because things were good for a long time and nobody thought we’d lose our rights if we stopped paying attention. Same story throughout history.
As for the people being fed misinformation, they’re just rubes. Our education system failed them and now grifters who don’t care about America are taking advantage of them.
And single issue voters, voters unwilling to push their own party (don’t vote or vote for a third party instead of getting involved in the party primaries), siloed media echo chambers, lack of critical thinking, dark money in politics, the proliferation of algorithm driven content feeds, 24 hour news cycle. Complacency is definitely a factor, but not the only one.