Because most fiction (including fanfics) doesn't include such explicit teachings that are applicable to one's life. Usually it's found in non-fiction, and didactic works of fiction usually must be subtle lest they be labeled "preachy".
The themes in and lessons of HPMOR are relatively uncommon in fiction.
Didactic fiction is a rarity in modern times, and writings that are both didactic and tell a story well are rarer still.
Didactic fiction is a rarity in modern times, and writings that are both didactic and tell a story well are rarer still.
I'm not so sure about this. Didactic and polemic works are uncommon (though not unknown) in genre fiction, but they seem less so in literary fiction; George Orwell is the first writer that comes to mind, but he's by no means the last. I've even heard didactic content described as a prerequisite of literary quality, though I can't remember where at the moment.
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.