Not all players are aggressive. The Gruenfeld is quite barky from the getgo, which some chess players like myself wouldn't feel too comfortable playing. Calmer, more positional payers like myself would probably choose another Anti-d4 system like the Nimzo-Indian or Queen's Gambit Declined. The Gruenfeld chooses to hand white the center and attack it later, while the Nimzo-Indian chooses to stop white's plan outright.