Yan Fei's bomb was targeted at the US Forces high-power radar base, a location critical to the naval base. Consequently, it was fiercely protected by various types of air defense weapons—numerous as hairs on an ox. Yet, Yan Fei's attack was so sudden that from the moment the bomb was detected to the time it hit its target, there was less than thirty seconds. The Americans had no time to mount an effective defense, and the radar station was obliterated by the ferocious bomb.
You can't say the American military bases' defenses were poor; on the contrary, they were very much in place. Consider that ordinary airplanes cannot get anywhere near a US Military Base and drop so many bombs, since the US Forces' layered defense would destroy them before they could approach the military base. Even if a plane did manage to penetrate, the US Forces would have detected it well ahead and had plenty of warning time to activate air defenses and ensure no enemy could drop bombs over their heads.
