I haven't found a definitive in-universe explanation, but based on what we know about blasters and slugthrowers in the Star Wars universe there are some reasons to prefer the former over the latter as a sniper rifle.
Note: Slugthrowers in Star Wars are depicted very similarly to real-world firearms, so I will assume the capabilities and limitations of the latter apply to the former.
A blaster is easier to use than a slugthrower for long range kills since the latter requires adjustment for bullet ("slug") drop, wind effects, etc.
Slugthrowers are difficult to use against long range targets because the sniper needs to take into account the ballistic trajectory of the slug. Real-world snipers often work in teams with a spotter whose job is to measure distance, wind, etc., to aid the shooter. The difficulty of accounting for a slug's ballistic trajectory is compounded by the fact that snipers in Star Wars would generally need to fight on a variety planets with a different gravity, wind levels, etc.1
Blasters, on the other hand, don't seem to have this problem. For example, this gif2 from Star Wars Battlefront (which is largely canon) shows a long range RT-97C blaster shot and incoming blaster fire with no observable "bullet drop":

The target was about 100 meters away (judging from the objective marker in view when the shot was taken). Here's an image2 with the objective marker circled; note the indicator (albeit barely visible) that the marker was 33 meters ("33m") away when the shot was taken:

It's worth noting that the developer, DICE, is famous for other first person shooters -- e.g. the Battlefield series -- which do have bullet drop. The lack of "bullet drop" in Battlefront is not a game limitation.
A sniper using a blaster is therefore more likely to hit his target, even if he isn't the best of marksmen.
Slugthrowers are ineffective against most armor in Star Wars.
Wookieepedia's article on slugs cites Star Wars Rebels: Rebel Journal by Ezra Bridger and notes that slugs are ineffective against Imperial stormtroopers' armor (which is not very effective against blaster bolts). A slugthrower sniper rifle would be useless against a stormtrooper since stormtroopers' armor covers them entirely.
A slugthrower sniper rifle wouldn't be much more useful against a person who was only partially protected by armor (e.g. someone not wearing a helmet). It would already be difficult to hit a person due to the slug's ballistic trajectory, and forcing the sniper to aim for an even smaller target like the person's head would make the shot far more difficult. Even real-world snipers don't always aim for the head. For example, Chris Kyle mentions in his autobiography American Sniper that he usually didn't attempt head shots:
It can take a few shots to put someone down, especially the drugged-up crazies we were dealing with in Iraq, unless you hit him in the head. And contrary to what you’re probably thinking, not all sniper shots, certainly not mine, take the bad guys in the head. Usually I went for center mass—a nice fat target somewhere in the middle of the body, giving me plenty of room to work with.
Chris Kyle, American Sniper: The Autobiography of the Most Lethal Sniper in U.S. Military History, p. 100
From a practical standpoint, slugthrowers are extremely unlikely to kill an armored target.
Slugthrower sniper rifles have muzzle flash and are LOUD, and thus not necessarily much stealthier than a blaster that fires a visible bolt.
You can use a suppressor to reduce muzzle flash and make the slugthrower quieter, but that doesn't necessarily help much. To give some rough numbers, Wikipedia cites noise measurements of suppressed firearms and notes that in general the noise is still >100dB and on par with other loud noises like rocket engines and sirens:
Live tests by independent reviewers of numerous commercially available suppressors find that even low-power, unsuppressed .22 LR handguns produce gunshots over 160 decibels. In testing, most of the suppressors reduced the volume to between 130 and 145 dB, with the quietest suppressors metering at 117 dB. The actual suppression of sound ranged from 14.3 to 43 dB, with most data points around the 30 dB mark...
Comparatively, ear protection commonly used while shooting provides 18 to 32 dB of sound reduction at the ear. Further, chainsaws, rock concerts, rocket engines, pneumatic drills, small firecrackers, and ambulance sirens are rated at 100 to 140 dB.
Moreover, some suppressors work in part by reducing the muzzle velocity to subsonic speeds (which requires even more compensation for additional bullet drop).
Blaster sniper rifles are more compatible with the other blasters in use, providing logistical benefits.
By using blasters for all small arms (pistols, rifles, sniper rifles, etc.), it is easier to train troops on how to use them effectively. If troops used blasters for pistols and rifles but not sniper rifles, snipers would have to go through much more training. Furthermore, slugthrower sniper rifles would be nearly useless in the hands of troops who were not trained as snipers (whereas such troops could probably use a blaster sniper rifle in a pinch).
Blaster sniper rifles are also more likely to share components, ammunition, etc. with other blasters. This makes logistics easier and can help troops on the battlefield make the most out of the (possibly limited) equipment they've got. As a real-world example, Chris Kyle alludes to the benefits of weapon compatibility in his autobiography:
The Mk 12 was super-easy to handle, and was virtually interchangeable with the M-4, which, though not a sniper weapon, is still a valuable combat tool. As a matter of fact, when I got back to my platoon, I took the lower receiver off my M-4 and put it on the upper receiver of my Mk-12. That gave me a collapsible stock and allowed me to go full-auto.
Chris Kyle, American Sniper: The Autobiography of the Most Lethal Sniper in U.S. Military History, p. 100
The logistical benefits of using blasters for all small arms could be crucial to winning a war. General Dwight D. Eisenhower (who knows something about winning a war) is credited with this quote:
You will not find it difficult to prove that battles, campaigns, and even wars have been won or lost primarily because of logistics.
1H/T Lèse majesté for this comment.
2Created from this Youtube video (start at about 3:17).