The primary reason for the Imperial Japanese Navy victory at Savo Island is the prewar training and doctrine of the IJN.

In the 1930s, the IJN was focused on gaining superiority at sea by fighting and decisively winning a large engagement with the enemy battleship forces as they had done in the Russo-Japanese war at Tsushima Strait in 1905. The other components of the IJN, like submarines, air groups and carriers, and cruiser forces were to play a supporting role in the battleship contest by causing attrition to enemy battleship fleets before the final contest between the lines of battleships.

For the Japanese cruisers, one of their important roles in the prewar battle plans was to launch torpedo attacks against the enemy battleships. Therefore, Japanese cruisers all had torpedo tubes and many of them had heavy torpedo batteries. The IJN recognized that this would need to happen at night in order to allow the range to close to where the torpedos would be effective. Accordingly, the Japanese cruisers and destroyers in the prewar period practiced night torpedo attacks at close ranges and high speeds with running practice torpedos. Additionally the IJN type 93 torpedo was a contender for the most advanced torpedo in the world in 1942, clearly outclassing U.S. counterparts in range, reliability, warhead and speed.

In contrast, a night torpedo attack was not a core feature of U.S. cruiser doctrine. Many of the later U.S. cruisers did not even have torpedo tubes. The U.S. prewar navy did not spend much time practicing night torpedo attacks. This makes sense, because is quite dangerous for ships to practice night maneuvers at high speed and torpedos were expensive machines at the time.

While the Battle of Savo island is not an attack by cruisers against the battlefleet, it was a surprise night attack in which the torpedo was the decisive weapon. This fit the Japanese doctrine perfectly. Accordingly, their ships were deployed properly and their admiral, captains, and crews were well drilled in this kind of fighting. They were calling the most well-rehearsed plays in their playbook.

The U.S. forces, while professional, were not expecting the Japanese cruiser doctrine to still work in the face of air power. Their response was improvised and poorly coordinated. Individual officers, aircrews, lookouts and so forth may have performed poorly in their roles, but it's not because they were unlucky or negligent. It's because they were improvising and unaware of what type of attack specifically they needed to be prepared to counter. Siting specific errors or scapegoats is a futile effort, since the fog of war will always create these.

The doctrinal superiority of the Imperial Japanese Navy is the root cause of the IJN's victory at Savo Island. Radar and increasingly competent air search would make this doctrine harder to successfully execute as the war progressed, but for the year 1942 it was extremely successful.

Source: David Evans and Mark Peattie Kaigun: Strategy, Tactics, and Technology in the Imperial Japanese Navy, 1887-1941