Why surface raiders preferred lone merchant ships

How would a surface raider use torpedoes against an unescorted convoy?

It could attack selected ships, but scattering and radio reports would make the encounter dangerous, so raiders generally preferred isolated merchantmen.

A raider encountering an unescorted convoy might work down the formation and use torpedoes against worthwhile targets, but the convoy would not remain neatly assembled. Merchant ships would scatter while several radios reported the attack and its position. That delay invites cruisers and destroyers to converge, the exact development a commerce raider must avoid. Historical raiders therefore preferred isolated ships on shipping lanes, as Graf Spee's fate demonstrates the danger of meeting even a small group of determined Royal Navy cruisers.