A fixed prompt likely was why I couldn't maintain it. The prompt was generic and open ended, but I tired quickly of coming up with new stuff to the same questions.

But the likely real reason is that it just became a chore. I probably should have reduced it to once a week or something. I have kind of tried that as well and still failed.

Another reason is quality. The first draft of anything sucks really bad, but I don't want to write badly. It's a lot more effort to write something present or future me will be happy with.

Ultimately, I think it's a matter of perspective and priority. I like the idea of journaling, but it is somewhat labor intensive (especially if the "first draft" is not good enough for you). The reality is I often choose to do something else with that time.