The main reason for this is because the CF is a reward for doing DB-related activities. Playing a game alone or with people outside of the club doesn't really connect to the DB at all. We encourage playing with members of our community by rewarding that activity.
Before we started awarding PVE games (games played cooperatively online, like The Old Republic and Diablo 3), CFs were awarded only for DB members playing PVP Star Wars games with each other (think Jedi Outcast, Jedi Academy, etc.). When those games started getting older and we had nothing new to replace them with that had the Star Wars theme, we decided to allow members to play other games--as long as they did so together.
This new rewarding style (PVE) was meant to relate to the old style by requiring at least two DB member participants. It was a compromise, and designed to prevent people from gaining DB rewards for simply playing with their own friends or playing solo.
Hope that clears things up.