Boredom.
Well, not quite that but, yes, I tried the outline and the creating of character backgrounds (actually that served me when when I was acting) and even tried for the arcs. It was, for me, so boring.
And when I was done with all of that I was done. Completely done with the story. That is, of course, with my own stuff.
I did write, many years ago, a script for some filmmakers in LA (whether it ever got done, I don't know, the check cleared the bank and that was all I was concerned with.) In any event maybe it's because it was someone else's story and characters that I didn't have a problem with it. But when it comes to my own stuff, once I do all that work, it's just done done done.
I read somewhere that's how Stephen King feels about it.
I have to admit, what I love most is the thrill of discovery. And, sometimes the backing up and redoing parts to fit what you're doing now, too. I'm a tinkerer. I'm also a lousy chess player because I can't stop myself from looking at the board and saying "let's see what happens when I move this piece." That's how I feel about my story. "let's see what happens if I do this."
I realize this is not for everyone. I do tend to encourage people to at least try it. If it doesn't work for them, nothing lost but a little time. I've tried several other ways of working, what the heck, they're all valid, why not try them? But when it comes down to sitting on the seat and doing the work, it's the only way I can work.