Following up on last week's post on Why RPGs Fail, I would argue that consistency of releases is the major reason. Even though people have complained about the design of D&D, saying it is too abstract, not realistic enough, arbitrary, the system has managed to remain viable for 30 years. Only recently has the Pathfinder system started supplanting it in some areas. For 30 years, support material has arrived for D&D/AD&D regularly as clockwork. The core rulebooks have only gone out of print at one time, just prior to the release of the third edition rules, in order to clear 2nd edition material out of the distribution channel prior to the release of 3.0.
Similarly, Shadowrun has remained almost continuously in print since release, with supplements arriving regularly, not as often at D&D, but still much more so than R. Talsorian's Cyberpunk, which let the core rules go for so long, despite a significant catalog of support materials, lost much market shart and consuemr interest because of the lack of the core rulebook.
Tunnels & Trolls, despite having been in print almost as long as D&D, had some success in the 1980s but since then has never come close to growing a large player base. Again, I attribut this to the lack of consistency of releases. While the core rulebook is still available, the company has release few supplements or adventures for it, in print form, for over a decade.
To put it simply, in order to make an RPG successful, a company needs to keep the core rules in print, so that customers can find it and a steady stream of product releases, so that players know the company plans on supporting the line long enough for them to get investing in an d keep playing it.