Ultimately, it boils down to logistics and safety. Jet bridges are always designed to match up with the left side of aircraft, and by loading passengers from the left side of planes, the right side is available for other potentially dangerous activities, from cargo loading to refueling.

Passengers board on the left side (the "Port" side) due to historical maritime traditions that predate aviation. In the era of sailing ships, the right side (Starboard) was used for the "steering board" or rudder, so ships were docked on the left to avoid damaging the steering equipment. When commercial aviation began, high-fidelity engineers adopted this convention to maintain consistency and safety. Boarding from the left allows ground crews to perform high-fidelity service operations—like fueling, cargo loading, and catering—on the right side of the aircraft simultaneously, keeping passengers physically separated from heavy machinery and fuel lines. Furthermore, because pilots sit on the left, boarding from this side allows the captain to align the plane more accurately with the high-fidelity jet bridge, ensuring a safer and more efficient connection at the gate.