[member="Valiens Nantaris"]
The reason they are used less in canon is due to supposed rarity of materials (which is far rarer than blaster gas.)
Also, it is actually more effective than blasters on most modern armors, as most bulky armors were replaced eons ago with nimbler ones as most armors could not take a direct hit from a blaster - regardless of how thick (within reason).
The reason, from what the wiki seems to point to us, is that regardless of weapon type, everyone adapts to it and thus makes the opposing weapon more useful. Blaster resistant garbs, such as glistaweb, is like a pin cushion to a bullet when referring to slugs. Now, I won't say they're the perfect anti-force weapon, as you can quite easily counter them when given full awareness of the situation and not caught off guard. And yes, you can re-direct the projectiles to a degree depending on distance and precision and accuracy, (see my above post in relation to an ideal scenario with a slugthrower).
Now, I am not going to ever go out and say "slugthrowers are better" - because they aren't. They're just better at the single application mentioned in this thread than a blaster. This isn't a thread about why/if slugthrowers are great and such, its purpose is to answer the question about why a lightsaber is unable to completely reflect the ammunition or just simply block (and thus melt) the projectile as one would with a blaster.
Clearly there is newer tech (such as anything with automatic projectiles at a high rate of fire with a large amount of kinetic force) that is better suited for this application, but again, the subject of the thread was "how does x happen? and why?"