If I might venture an answer to your question, I'd say it's about the personality of the stories here. What attracted me to this site was how, overall, most of the stories here focus on plot. Long entries with good narrative and interesting character development are encouraged, and nobody seems to mind if you focus more on plot development than on...other stuff.
In Addventure, ideal episodes are relatively short, and if there isn't a blow-job, shemale action, or what-have-you every other paragraph, people lose interest, and the writer is left to continue it on his/her own.
Plus, the sign-in thing does turn away most casual contributors.
Other possibilities:
The lack of set options might scare people with the whole "blank page" paralysis thing.
Most of the pictures no longer work, and considering how many people went a little crazy with them when they were introduced, that leaves a lot of holes.
