Think about when flossing is supposed to happen: the end of a long day, when you're tired and just want to get to bed.
Now think about what string floss asks of you: wrap it around your fingers, work it carefully between each tooth, try not to snap it into your gums, spend several minutes doing something that's uncomfortable at best and painful at worst.
It's the most demanding step in your routine, scheduled at the moment you have the least patience for it. That's not a willpower problem. That's a design problem.
One online commenter put it perfectly — they said flossing demands dexterity and effort right when you have none left. Most people don't quit because they're lazy. They quit because the tool makes it too hard to say yes at 11pm.
The fix: Remove the effort. A water flosser replaces the wrapping, the snapping, and the technique with one button. You fill it, press start, and guide it between your teeth. The whole thing is over before you've had time to negotiate with yourself about skipping it.