May I propose possible reasons to the decline on the forum? Perhaps...
- Problems with the Tube upgrade plan mean people hate the tube more (weekend closures etc)
- The forum's been around for nine years, and in this time, there's only been one really major change to the tube, namely the loss of the East London Line
- Most people only attempt one challenge, it's the Full Network, and people usually attempt it just once
- Wikipedia does not mention the existence of a forum, or the existence of alternative challenges
- The new blood you need is students/the just graduated, but most students/recently-students are more interested in taking selfies
- The original members of the forum are only growing older, and I would assume the march of years makes tube challenging more difficult
That said, I guess some people on the forum are a little off-putting, including:
- Geoff and Andi, the two most famous tube challengers of modern times, used to take digs at each other all the time before Geoff stopped using the forum on getting his second record, which can seem immature. Andi's posts still remain, and one or two of Geoff's posts also remain (those which have not been converted into "trains")
- Rhys B, the youngest member of the forum, shows strong political views which can put people off
- Every few months, GuyBarry gets pernickety over one of the Quizzes and Games, creating a huge argument which is finally settled by Tubeguru either blowing his top or almost blowing his top
- Moley's signature lists himself as "Bad Cop" and Tubeguru as "Good Cop", leading prospective forum members who think Tubeguru is a grumpy old man to believe that Moley, who does not often post, is even worse, so they don't bother signing up
The rest of us have our moments as well!