Answer:
Because the United States wanted to expand into the Pacific as well.
Step-by-step explanation:
To really explain this, we must go back to the 1937 Second Sino-Japenese war. After what Japan did to the occupied lands, and after they took French Indochina, the USA placed an embargo on Japan and stopped sending it oil unless it returned Indochina and its land in China to their former countries. Japan didn't have this, and also had years earlier started taking over islands in the pacific, which clashed with you guessed it, the USA. So what's the logical choice for Japan, accept the much more powerful USA's demands, or bomb Pearl Harbor, and risk war? But wait, the USA are Isolationists, they won't join the war, let's bomb Pearl Har- oh wait they joined the war 1 day after that...