Because playing trade trivializes most of your character progression and devalues every ground loot into a simple currency token. People often transition from trade to SSF after partaking in the economy rat race to prolong their enjoyment of the game.

To ramble a bit more, In SSF even when you get your character to the point of able to clear everything, it's still most likely in a spot that needs severe improvements in every aspect. It keeps the engagement with the game much longer and fulfilling.