I would imagine part of it is curiosity. Additionally, people reading the forums can't see attached images unless they are registered and signed in (at least I can't). So... they register. I would also imagine that most of the new registrants aren't collectors or serious flat painters. They simply paint figures and flats are another type of figure. They may be looking for something in line with their personal interests - Orcs and elves, Napoleonics, ACW, Vikings, etc. If they don't see that, then maybe their interest wanes a bit. Also, people new to flats are going to be a bit shy when it comes to posting their own work when they compare it to long established painters. This is a trait shared with round figure painters, plastic modelers, etc.
That said, the open forums are a bit thin on how-tos, tips, techniques, etc. Most of the completed flats are simply a pic and a 'here it is'. Many are posted without a scale/size, manufacturer, paints used, source, and/or commentary about any issues with the piece. There is little apparent effort to attract and hold new members who just want to learn how to paint a decent flat. Maybe it's time to up the game. The next time you post your latest piece, state the above. If there's a casting or engraving flaw, say so. Then tell everyone how you fixed it. Don't be afraid of receiving feedback - good or bad. And, limit the feedback to what was asked for - not the genre, subject, etc.
One suggestion, when people register for the site, perhaps a field (or fields) for interests, genres, scales, painting mediums could be included and be visible to everyone. That way people could find others with similar flat interests.
I am curious about the six pages of registrants and their average age and where they live.
Cheers,
Glen