The distance between Europa and Ganymede, even at closest approach, is nearly 400,000 kilometers: this corresponds to a round-trip lightspeed delay of 2.5 seconds. That's best case: when the moons are opposed this can be over 11 seconds. If you've ever had trouble on a satellite call with less than a second delay, you can imagine the frustration of holding inter-moon conversations in the Jovian system.