makes me wonder why almost all everything that has potential ends like this...
Because the better something is, the more work you need to put into it to keep standards up.
There is something I call creator passion, which is the drive the creator has at the start of the project. As the creator works on the project, slowly but surely this passion dies down, turning what was a fun project into a chore. This passion can go up if good things happen, like people posting encouraging comments and the creator feeling satisfied by his results, but it can also go down faster if the creator feels frustrated by his results, by how hard is this work or if there are people putting him/her.
Anyway, the harder the project, the longer it takes, and the longer it takes, the higher the chances the creator's passion runs dry before finishing it. Once the passion runs dry, it requires a huge sense of responsibility and perseverance to finish the project, since what was fun is now a pain in the *ss, and anything and everything becomes a good excuse to stop it temporarily or permanently.
The worst part is that if the creator stops for a little while, he/she may end up feeling guilty (like me, lol), which may make him/her afraid of coming back and facing the backlash from disappearing, which makes it harder and harder to come back to the project, even if you feel like finishing it again after the rest.
Well, those are the main reasons on my point of view. There may be other reasons like disease and real life or other personal reasons which I don't know. If you agree, disagree or have something else to add, feel free to reply.