When many combat factions, have their own supply fleet or just use have members fly indy's to get materials there is probably little incentive for many players to join OF.
In fact probably more disincentives as using an OF tag is an advert for piracy and you will not suffer any real consequences if caught smuggling or bending ID rules.
There are still a number of trading factions on the tracker maybe those who want this way are saturated with current population.
Maybe make all >3600 hold trade graft run a corporate banner?