A crossbow is something the Ewoks definitely have the materials and know-how to craft.
But I think you’re reading too much into the examples given and the choice of Ranged (Light) as the skill. It gives four examples, three of which are one-handed. The only outlier is the bow, and a GM would be well within his rights to simply change the skill to Ranged (Heavy) if the narrative is of the “simple projectile weapon” as a bow.
The differences between a crossbow and a rifle are many, but there are a couple specific reasons why “point-and-shoot” isn’t a sufficient justification.
It’s just an upgrade: It isn’t that they can’t use the given item, just that they don’t have a good understanding of it. An upgrade is a relatively minor difference.
Mechanical understanding: A crossbow is very obvious, and works on known principles. Pull back string, flex stick, secure. Release string, stick flexes, arrow shoots.
A slugthrower, however, is not easily understood by someone without an understanding of how it works. You pull the trigger, there’s a bang, and then there’s a hole in something relatively similarly located to wherever you were pointing the gun.
Then there’s the safety, select-fire modes, bullet drop, you can’t see the projectile, did you load the magazine correctly?, did you do something that jammed the weapon?, are you prepared for the recoil?, etc.
With a blaster, you eliminate several of those objections, but you still lack the fundamental understanding of how it works. Do you know to replace the power pack, or how to? Do you understand select-fire? What about the principles of aiming a blaster? The sights are very different than those on a crossbow. And what about optics? Do you understand how to use an optic?
Any of these would probably be enough for an upgrade. In fact, several of those are good arguments for upgrading powered weapons, though that should be on a case-by-case basis.
All of these can be overcome, it will just take some training and practice, and that’s a narrative explanation you were looking for, isn’t it?