In-universe:
Though the rules of maneuverability in space are different than in an atmosphere, it would be logical to design control systems for spaceflight (and especially space combat) that would mirror the control system of an atmospheric craft, on which pilots would have gotten their first flight experience (as atmospheric craft would be more forgiving of failure than a spacecraft).
Many ships, including the Millenium Falcon, are designed for BOTH atmospheric and space maneuvering. Again, it makes sense to make the controls for both maneuvering systems (which would be quite different) appear to work the same.
Reactionary control systems (thrusters) that had the same size/power of thruster everywhere would naturally result in different X/Y/Z-axis rotations rates, based on the shape of the craft. Specifically, the rate of movement is a combination of the distribution of mass along each of the axes, and the length of each axis allowing thrusters to gain mechanical advantage in torque. The Falcon would probably pitch faster than anything else, as that's the longest dimension and thus the thrusters can be placed for maximum advantage. Rolling would likely be next best as the moment of inertia would be similar, while the width of the ship is the next longest to its length. Yawing would probably be the worst, because despite being able to use thrusters placed at the extremes of both of the long axes, around this axis the ship has the highest moment of inertia. Someone piloting this ship would thus use the maneuvering thrusters to their greatest advantage, by rolling to place their destination above them and then pulling up.
RCSes that had DIFFERENT thruster powers for each axis would likely focus on the axes the pilot was most used to maneuvering with, while compensating for axes in which the ship had an extreme disadvantage due to shape. Back to the first two points, a pilot who learned on a Skyhopper or other atmospheric craft would probably focus on pitch and roll.
Out of universe:
The original Star Wars Death Star battle was patterned heavily after a real-world attack, the "Dambusters' Raid", which required slow-flying, less maneuverable light bombers to negotiate a valley leading up to the dam, at which point they would aim for the base of the dam or other structural weak points. This was while trying to deal with far more nimble fighter craft getting in behind the bombers and chewing them up. As such, the designs of the ships in the Star Wars universe, and the way they flew, were patterned on real-world aerodynamically-maneuverable planes, because the drama of the real-world raid would be lost if an X-wing could cut its engines and spin 180* to fire backwards at the TIE fighters while still moving in the same direction it had been pointed previously.
Also, as an audience, we're used to seeing craft identified as "fighters" maneuvering mostly by rolling and pitching. You gotta give the audience what they expect, unless your aim is to specifically give them something they don't expect (as in the more real-world maneuvering capabilities of, say, a Babylon 5 Starfury; there are several scenes where the intent of the writers/director is to surprise the viewer with how the craft can maneuver, mirroring the surprise of the pilot who's just seen the Starfury go from in their sights to behind them lining up a shot).