First, while a Wiki can be made to work for what are basically margin scribbles, it's not an entirely natural fit. Here's the last line of Catullus 48 marked up —
sit nostrae <sch lemma="seges segetis" grammar="f.">seges:: crop, grain field.;;</sch>
<sch lemma="osculātiō ōnis" grammar="f.">osculationis:: kissing.;;</sch>. //
That's sort of messy, though it gets the job done. Other people have been able to use the system without too much difficulty, though sometimes with rather different style habits than I favor.
There are probably other ways to handle text annotations like this that are a lot more natural for people who are classically inclined but don't have my background in computer programming, publishing, etc. But I'm not sure that would result in more people adding to the site, which leads to the second thing I believe has kept submissions slow — annotating a text well is a huge pain in the ass. Some people even mentioned this when I made the Scholiastae announcement. The technology isn't the hard part. The research, the double checking, the hunting down citations, the worrying about what really needs comment and what does not — all these things are a much bigger commitment. Even a simple 10 line poem by Mimnermus for Aoidoi will take me a week or two, which includes checking up on different versions of the text, as well as dealing with comments from the small group of people I can send out early drafts to (who probably deserve some sort of award). Someone has to be very committed to a text to mark it up formally and carefully in any medium at all, much less on a Wiki run by someone you've never met.
My first thought on putting up Scholiastae.org was that I'd give it a year or so, and if no one was interested enough to publish on it, I'd just close it down. But I still find it useful as a dumping ground for my own purposes. It's the best place for me to keep notes on prose works, but I've found it a nice place for smaller poems, too, like things from The Greek Anthology or the fragments of Pindar. Besides, I can hope someone else will find it interesting enough to add comments to eventually.