I can tell you from experience in co-founding RTR that TWC was simply the place with the greater number of modders, which is why that effort happened there and why EB decided to open a second forum there. In order to attract new talent having a strong presence at TWC is necessary.

Forum-goers may have a choice and it may be personal preference, but mods have little unless they want to miss out on the opportunity of a greater base of modders.