Crossbows have the advantage of a stable base, easier sights (Although that could be due to familiarity with firearm sights), less strength required to maintain tension, etc. Which basically equalled less skill. Oh hey, ain't that why the crossbow replaced the traditional bows by sometime in the Medieval period. And then came the firearms which just opened a whole new game to the field. Wait...let me try that again...
Looking down a sight is indeed pretty damn easy. All you have to do is train a bit to gage projectile drop.
When I went out target shooting with my shotgun for the first time, I thought I'd miss and it was obvious my grandpa wasn't convinced I'd hit it on the first shots (at least, didn't look like it). Didn't miss a single shot, and I really can't understand what would be so difficult about it if you grasp the concept of the iron sights. Just line 'em up. Hell, I hit my first shots while closing the wrong eye! That did give me a good kick on the nose, though. XD
The only "difficulty" is getting enough pellets precisely where you want them, but that's just random once you know what the scatter is like. Crossbows, using the same sights, should be the same I would expect. Bows, on the other hand, require much more skill.
Easy to train people to use them, and devastating penetration power. No wonder people switched to crossbows despite the church being against it, and then to firearms as soon as it was possible.