The construction of a crossbow is not simple -- neither is that of a good bow, btw -- but it's a lot more simple to use a crossbow than a bow.
Bow: insert arrow, tilt the bow just right, pull out, feel the entire draw weight in your arm and shoulder, aim quickly (before you start shaking) and let the string roll off your fingers. Practice several times a week for several months to master the bow.
Crossbow: draw string till it clicks. Insert arrow. Take all the time you need to aim. Pull trigger.
Add to that that crossbows can have much higher draw weight than bows, while the bolts themselves are shorter, which results in a much faster projectile, effectively creating a more stretched firing arc -> aim more directly.
All in all, it is a lot easier to hit a target with a crossbow than with a bow. It requires a lot less training and devotion to the weapon. Which is why d20 / D&D 3E correctly classified it a Simple Weapon.
In Conan, hunting bows are classified as simple weapons, which is arguable. All bows are equally difficult to master and should be classified as Martial weapons.
Effectively, those classifications only make a difference for the Scholar and Thief classes, and not an overwhelmingly large one at that. Change it, leave it, just as suits your campaign best.