Because after a decade of doing nothing about Japanese aggression in Manchuria and China, FDR didn't care about Asia.
Only after Germany attacked Russia did FDR move quickly to secure Stalin's flank by cutting off Japan's oil supply thereby forcing the Japanese to secure alternate supply from SE Asia.
The Japanese attack at Pearl was intended to prevent a significant American response for about a year.
The Japanese attack at Pearl was intended to prevent a significant American response for about a year while their thousands of Emperor-worshiping citizens already in the U.S. went to work at sabotage and spying...
However, FDR totally cut their strategic balls off when he interned most of the Emperor worshipers and eliminated virtually all of the planned spying and acts of sabotage...
The nips not only counted on the American Pacific Fleet carriers being in port, but they counted on their sleeper spies and saboteurs being in place and ready to do Tojo's & the Emperor's bidding...
The sad part is that here we are 75-years later and the Nation's two (2) major enemies (ChiComs & moslems) with a desire to destroy America each have hundreds of thousands of their citizens/sleepers firmly implanted within American society, industry, academia, and the state & federal governments...
Nothing has been learned, from history, by any of those who rule us...