Why don't more people use Git and Markdown for their todo lists and/or GTD systems?
The Git client is ideal for displaying lists and editing a little text file you git push could not be more simple.
This group of people is willing and ready to learn but you still often hear "just checkout the project again" when a problem is encountered.
Most people just use an app and are fine with that functionality. My partner and I use a Google Keep list that is shared between our accounts. Live editing included and it does everything just fine.
There are more elaborate app solutions out there that do more and solve most of the problems.