Thanks for input. Let me upggrade with Austro Hungarian - Ottoman conflicts.
When the Ottoman conquered Serbia (decisive battle held at Kosovo 1389- led to crash of Serbia, and Otoman rule of region for next 5 centuries) - this resulted in Serbian exodus to the west in following period. They settled in areas around present Croatian / Bosnian border. As Austro Hungarian empire was under full threat they took the Serbs and Croats in bordering region, and supported their stand giving them some benefits in terms of economy, autonomy, but mostly - they kept them well armed in order to keep the border.
The terrain is hilly, mountains, ridges, creeks, rivers, woods and without much "old fashion" fields to develop ordinary field tactics in terms of line warfare, which was one of the first conditions to make different type of military engagement.
Second, the people there (both Croats and Serbs) by character prefered and developed their own way of fighting - by ambushing from concealed positions, hills, woods, etc. Witout much care of the military ethics adn doctrine of the time.
Thirdly, Otomans had much stronger force and in order to keep the border - the lighter - defending force used the guerrilla tactics to keep stronger force engaged.
At open field this would not have happened.
The people keeping the border, with given autonomy in decision making and Austrian (and venetian) support were not the nobility, but commoners - thus there was no much ethics involved in whether it is ethical to target the officers or not, or to shoot from covered / concealed position or not - so they just acted the only practical way.
In the same time the usual ethical military doctrine, developed by nobility was somewhat different, but for some reason the Austrian court turned the blind eye as long as the border was kept, and the Austrian nationals did not really fight, except in the role of logistics, advisory role, military observeres etc.
As far as the last of 19th century wars were kept elswhere in Europe with old generals running the show, it is interesting to note the further development of rifle as a main military weapon.
Under such influence, famous mauser 98 was developed in longer version around 1898 which met the ww1 of 1914. The lenght of rifle was determined by the last wars in western Europe kept in line where long rifle is needed when firing over forward line of kneeling soldiers, the idea already obsolete by year 1898.
The ww1 has proven the weapon too long to be practical, thus it was shortened between the wars, and shorter mauser 98k (karabiner) was designed to be the main Wehrmacht weapon of ww2.
ww2 has shown that 7,92x57 ammo was to powerful, and too heavy - which also means the rifle was heavy. Which also means less ammo for soldiers to carry.
Average range used was up to 400 meters, so there was really no need for high power rifle.
Thus, medium power ammo evolved, first for german SG44 (sturm gewerhe 44 - german automatic assault rifle, seen first action in German para forces liberating Mussolini from capture) - in same caliber, same shell but with reduced load.
The next development with lessons learned after ww2 in medium power rifle and bullet was 7.62 x 39 - russian in semi auto rifle, and well known ak 47, and american 5.56. Designed for average ranges determined fromww2 experience.
Hi power rifle / ammo kept their practicability and use only as special and sniper weapon ever since.
And thus, the history, lessons learned and practicality and effect kept the role of sniper ever since.