They are obvious and designers were aware of the benefits of centerline positioning very early. One need only look at the early turret monitors to see that. Even most predreadnoughts had their heaviest guns positioned at the centerline. The two questions somewhat go together actually as noth were limited by a mixture of technology and inertia.
The main reason why superfiring turrets of uniform caliber were not used until the USS South Carolina was that designers were worried that the stress on the ships structure and the lower turret would be too much with a superfiring turret and cause the ship to crack and rattle itself apart. Its important to remember that while engineering was quiet advanced at this time, accurate modeling was still quiet difficult particularly for things which had not been done and thus not yet been measured. Thus even though many designers realized the value of superfiring, it was difficult to get plans approved. No officer wants to be responsible for approving the design and building of an enormously expensive ship which blows itself apart. Caution keeps careers after all, with the great advances in steel technology and some daring this was eventually overcome with the South Carolina class having the first superfiring turrets of equal caliber. The worried issues about cracking were mostly unnecessary, but as an important context it's useful to remember that just 30 years prior when iron hulled ships were becoming the norm and both modeling and metallurgy were not as good several ships were lost to design errors in newer designs usually ruining the career of those involved in a scandal, the most famous example that comes to mind being HMS Captain. So admiraltys like any bureaucracy were hesitant to try new things which might end in scandal and loss of life, and great expense(battleships in particular often represented a really major national investment sometimes involving civilian funding drives).
The reason for all big guns ships happening when they did is somewhat similar in regards to it being a mixture of when technology evolved and evidence overcame bureaucratic inertia.
Since the evolution of gun based naval warfare up until around the 1870s or 1880s naval battles had happened within close range guns being sighted basically by naked eye and thus the maximum range of a gun was of little importance. Especially when propulsion was reliant on wind, even if you could theoretically have longer range guns and try to sit outside of your opponents range, there was no guarantee the wind would make this strategy possible. Ship designs(for battleship atleast) thus focused on weight of shot, figuring that if you were going to fight you needed to be able to close with the enemy and sluggish it out. This is how you get huge men of war with 120 or more relatively small guns like HMS Victory or the Santisma Trinidad.
This would change slowly with the adoption of steam power and shell firing guns first making artillery much more damaging to ships and ships much more maneuverable. This is why the ironclad frigates tended to have far fewer guns than similarly sized sail ships, with HMS warrior carrying only 48 even though compared to HMS Victorys 102 guns even though warrior was 2.5 times larger than victory. Quiet quickly after early ships like Warrior the arrival of turret monitors would fully transition away from the broadsides of relatively small guns in favour of large turret mounted 12" and greater guns on ships like the Italian Caio Duillio(1873) which carried nought but 4 17.7 inch guns.
The turret battleships evolved as engines, armor, and turret technology got better into the Pre-Dreadnoughts type which fought at Tsushima. Keeping the main battery of two fore and two aft heavy guns, most predreadnoughts carried an assortment of lighter guns from light 3 inch anti-torpedo boat guns to 9.5 inch secondary batteries meant to throw a lot of shot at closer range. The designers of the day still figuring that fleet engagements would happen close enough that the much greater rate of fire, maneuverability, and lower cost of lighter guns would overwhelm the expensive slow firing heavy guns at close range. A particular fear was that ship with too many heavy guns would be unable to stop smaller faster torpedo armed ships, who would be difficult to hit with pondering slow to reload 12" guns.
Three major technological innovations would smooth the path for all big gun ships. First rapid advances in rangefinders and academic training of gunners in the 1890s would allow big guns to actually have some chance of hitting target near their maximum range. Second, improvements in hydraulics and turret design increased the rate of fire of bigger guns. Most importantly though was engines, where rapid progress was being made in the 1890s leading to much more efficient designs, the advent of naval turbines in particular was a major shift. These advances along with the retirement of much of the naval brass whose careers at sea had been aboard old broadside ships opened the way for all big gun ship designs being seriously considered and debated pre-Tsushima with several ships being officially considered and designed by 1904 in Italy, The US, Japan, and Britian.
The battle of Tsushima though definitely sped along and gave creedence to the claims of all big gun proponents. The importance of speed became apparent as Japanese ships were able to decisively exploit better gunnery and rangefinding scoring numerous hits while far enough away that the Russians weak on that front had trouble landing hits. It showed that lighter guns were of little use in battleship to battleship combat as the ship with heavier guns could now exploit its longer range effectively using speed to stay in a sweet spot where it had fire superiority even if the other ship had a higher theoretical weight of shot. As a side note tsushima also lessened worries about the vulnerabilities of battleships to torpedo boats on the high seas.
Thus with better technology and the efficacy of all big guns ships having been shown the world's navies rushed to design and build big gun battleships able to take advantage of the fire superiority that speed and accuracy at range had given them over older mixed caliber ships who could be kept from ever getting close enough to utilize most of their guns while exposed to the full firepower of Dreadnought and her cousins.
Superfiring was at this point aswell an inevitability. With speed of huge importance, it allowed ships to deliver the same broadside with less weight allowing greater speed as a superfiring ship like South Carolina could deliver the same broadside with 4 turrets a non-superfiring ship like Dreadnought or Nassau needed 5 or 6 for. It would take only 4 years after the commissioning of HMS Dreadnought for the US to commission the USS South Carolina and after it showed conclusively that superfiring was not really as damaging as had been feared most other navies adopted a centerline superfiring layout afterwards with the only exceptions being generally ships that were too long into the design process to change like the SMS Tegethoff.
Ps: apologies for errors, writing from my phone on subway.
Sources:
Gröner, Erich (1990). German Warships: 1815–1945.
Halpern, Paul G. (1995). A Naval History of World War One.
Beeler, John. (2001). Birth of the Battleship: British Capital Ship Design 1870-1881.