Blaming voters for not being enthused about picking between two genocidal options is a one of the greatest losing strategies of all time. Nobody should be surprised at all that a chunk of the Democrats' base didn't have it in them to hold their noses tight enough for something that utterly vile — least of all Biden & Harris's campaigns.
They should have known. They were given opportunities to learn before the election even hit. They ignored them.
It was a massively idiotic move that either campaign could have avoided with a stupid level of ease, and they chose not to. Voters that didn't vote did so because they believe mass-murder is bad; whereas the best-case scenario for the Democrat's campaign is legendary incompetence, and the worst is outright genocidal malice and greed. If you're going to get mad at one, I recommend the latter.