>This is why you don't see present-day slugthrowers being used, but much more advanced slugthrowers with higher ammo capacities such as the flechette rifle have been conceptualized, so this isn't by itself enough to explain the scarcity of slugthrowers.
I'll admit to not knowing a whole ton on this topic, but my best assumption would be storage and cost. From what I understand, flechettes are generally larger than bullets, and obviously are much larger than blaster packs. Because of that, I would assume that you would have to carry pounds and pounds of flechettes to get anywhere near the amount of shots provided by a single blaster pack. In addition to them taking up much more space, I would assume that producing 100 flechettes would be more expensive than 1 blaster pack, for what would end up accomplishing the same goal in all but the most specific of scenarios. That's not to say that flechettes or slugthrowers in general wouldn't have their uses, they definitely do, but the general idea would be why pay more to produce and store something that's going to do the same thing as a much cheaper alternative. And even in the case if Jedi-hunting, it could be argued that it would still be less expensive to arm 100 men with blasters and have them take their chances for what would cost much less than 100 men with slugthrowers. And when you get to thinking past an individual scale, and on larger scales like that, you get higher rates of success with larger groups with slightly inferior weaponry than with much smaller groups armed with slightly superior weapony.