Actually, it is only because the pressure refueling rigs have bigger hoses and pump at higher rates.
Overwing/open/gravity fueling is limited by the ability to handle the unrestrained end of the hose, as well as the danger potential of fuel spills. With a closed system, these limits are nonexistent.
Automation in tank vents, level switches, and shutoff valves allows pressure refueling with high flow rates, and the closed nature of the system helps prevent fuel spills.