Why were the japanese so successful when they attacked pearl harbor?

The attack was well planned, well executed and caught the US by surprise.

Japan had been at war with China for several years and had a fairly large number of highly trained and experienced aircrews from which the elite was chosen for the attack. This elite force was able to maximize the damage inflicted on the allies for the size of the committed force for months after the attack.

Japanese intelligence was excellent concerning the layout of Pearl Harbor and the location of ships within it. Literally, they were able to plan in detail and rehearse the attack numerous times to get it right.

Japan's intelligence information on Pearl Harbor allowed them to make innovations that proved vital to the attack. For example, they knew Pearl Harbor was too shallow for their standard aerial torpedo, so they devised a modification to keep them from going too deep when dropped. "Battleship Row" in Pearl Harbor included an inner and outer row of ships, the outer row could be hit by torpedoes but the inner ones could not. The Japanese fitted fins to armor-piercing shells and used them as bombs for attacking the inner row of battleships.

The US was just installing radar at Pearl Harbor and as yet had not developed a system for reporting and intercepting incoming aircraft. The attacking force was picked up on radar and reported while still well out to sea. The junior officer receiving the report famously told the radar operators to "not worry about it" since he assumed the incoming force was 12 B-17s flying to Pearl Harbor that day from the mainland.