Honestly people wanting to make a hobby project from scratch and then brag about it in a blog post to make themselves look like a more savy programmer/techie (hey, not blaming people, when it is done well it is way better than a resume with bullet points) is primarily the reason they dont use org mode. It is like wondering why people go through massive amounts of efforts to bake stuff from scratch when there is a super market and bakery down the street, except it is also like I said a resume.

If you want a plain text organizational, compositional and scheduling tool that you can use for the rest of your life and know that 30 years down the line it will be actively supported, developed and you will be able to tweak anything you want.... emacs/org mode is by far and away the best choice. It isn't even remotely close, we are talking about the difference between a planet and a tiny asteroid when you compare org mode to other plain textish organizational, compositional and scheduling tools.

For as long as humanity doesn't collapse and probably even after it does org mode and emacs will be used (there is going to be some nerd somewhere using org mode and ledger cli to meticulously track how many smoked rats and cockroach kebobs they have left to eat before they have to leave their bunker), there is just such an intense critical mass of utility under an open source license.

About the only bad thing with org mode at this point is it has such a vast nexus of interconnected capability it is difficult to even describe what exactly it is anymore lol. Like how would you even adequately describe org mode in a single sentence? It would have to be a Thomas Pynchon style sentence lol.