Once my teeth are brushed, I want my entire mouth to feel completely clean. I don't want to still need to go back to properly finish the job.
For me, brushing my teeth has always been a final act. It's the last thing I do before I get into bed. Trying to do anything after that would likely only serve to mess up one of the parts of my routine that is working for me.
(Sudden idea: brushing my teeth as the last thing I do before getting into bed, consistently, will likely become embedded into my subconscious awareness. Executing that behavior will help train my brain to think "bed" and "sleeping time" whenever I do it.)
Additionally, it is usually easier to convince myself to brush my teeth than it is to floss. If I brush first, it's a lot easier to skip over flossing. If I floss first, I won't skip brushing afterwards because brushing is the easy part of it.
Having double-threaded floss sticks helps. I'm less likely to tear the floss than with a typical string of floss. They don't cut off my circulation when I use them. I can get the floss into and out of the annoyingly small spaces of my teeth with far less hassle and frustration.
Additionally, using an electric toothbrush is essential. Personally, I am not capable of fully cleaning all parts of my teeth using a manual toothbrush.
It's a much better reward to feel like my teeth are properly cleaned and polished after finishing brushing than it is to put in effort and still find find rough spots on my teeth where I didn't manage to scrub away all of the plaque.