I do believe that people don't join trading factions for two main reasons:
1. They believe that the role play that goes along with character will slow down their money making. After all, there's not many of us whom only fly transports because we think they are fun.
2. The limited trading places. When your tied down to a faction there's only certain places you can go and the ID's are limiting. However, with the indie trader ID or freelancer ID your free to trade virtually wherever and whatever you want.
There is more variety in trading if your not part of a faction. People are always looking for more freedom, it's a natural thing.
