Because as soon as you hear the word Dutchman -- which is an archaic word, nobody calls Dutch people "Dutchmen" anymore -- but as soon as you hear Dutchman, you get this picture of the kid with his finger in the dike, preventing the thing from falling and flooding Holland, and so on. And so the entire issue is detoxified by the use of "Dutchman."
なぜかというと ダッチマンという言葉を聞くと -- これは古い言葉なので 今はオランダ人のことをダッチマンとは呼びませんが -- でもダッチマンと聞くと 少年が堤防の裂け目に指をつっこんで 決壊してオランダが洪水になるのを防いでいるような イメージが浮かぶでしょう