Before I start let me note that in the end this particular group of people didn't affect the election. Harris is on the way to losing all swing states. Her failure is much deeper than Gaza policy. Blaming anti-genocide voters for this is just copium.
With that out of the way, you can divide people with this position into two groups: Arab Americans and everyone else. Arab Americans are people who are feeling the genocide firsthand. So, obviously, they tried to appeal to the Harris campaign and get them to move from Biden's position on the topic. The result: They were either ignored or antagonized by Harris. That led to the abandon Harris campaign in Michigan and elsewhere. Harris considered those people acceptable casualties in her failure of a campaign, and so they were burnt out and the momentum behind the Uncommitted movement and others turned from "let's save our Palestinian brothers" to "fuck us and Palestine (because let's face it, that's basically what Harris was saying)? Then fuck you too". Harris thew them under the bus and was thrown under the bus in turn. Maybe not very logical, but a very predictable reaction. Harris treated Arab Americans with just that much contempt, and then she and her enablers had the gall to tell the people attending a funeral every other day to "shut up and vote for her".
Now as for everyone else, it's a more simple instance of taking a stand against a politician for doing something you cannot accept. Now there is a pragmatic idea here that if you allow the DNC to get away with this they'll think supporting genocide actually wins elections, or that their electorate are such pussies that it doesn't matter what they think. Add in the goal of pressuring Harris to drop that policy that was important at the start of the Harris campaign and of course the idea of not wanting to vote for genocide and this was the result.
Of course it's not all 100% logical, but there is logic here beyond "omg bad guy I no vote".