Yeah it's up to the artist, TGC can't make them do anything. MP22 was rejected because of it's 18+ content, and in fact is currently under review by the ESRB, I had to take the Youtube videos down. So I have no idea where that will go.
I've uploaded ALL of my model packs to the store(which was an ENORMOUS amount of work), except MP22.
But the fact of the matter is, it's hard for creators
of quality models to recoup their time costs selling in the store alone, where users cherry pick what they want. Selling them as a model pack, you will eventually earn a profit if the pack is quality and popular.
Fletcher and others might just be waiting until they've earned back their initial time investment before uploading them to the store, as this would kill their model pack sales.
----------------------------------------
"bond1 - You see this name, you think dirty."