Well, the japanese had several hundred merchantmen. They had ~100 DDs, and before 1944, very few dedicated escort ships (early kaibokans, subchasers, etc).
Given that almost no fleet DDs escorted convoys (many actually carried no depth charges at all early in the war), that doesn't leave much room for escorting, particularly since they had no naval officers at all assigned to convoy duty until 1943.
One possible reason is that over 50% of patrols out of Pearl in 1942 were to Truk. That's the "front" and near Truk everything is escorts---almost, unescorted is a rare exception in that area).
tater
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