Very interesting question. I am certainly not a Biblical Scholar or theologian, but several historically based issues come to mind. (1) For a long time illustrations of Jesus tended to reflect the late medieval style in art and also the style or styles dominant in Catholic Europe (olive skinned Mediterraneans, perhaps).
(2) I am not personally familiar with the pictures in early protestant Bibles, most of which were originally printed in Germany, many thanks to Guttenberg and his students, but it is safe to assume that they probably reflected Germanic physical types since they were being printed and distributed in Germany and other countries (Scotland, England) which would soon be protestant.
(3) The real key might lie with the King James Bible, KJV, published in 1611, in English which would have been the Bible most Protestant settlers of North America would have owned. (between 650 and 1800 about 75% of the immigrants to the US were protestant) So whatever pictures of Jesus appeared in the KJV, probably Germanic in appearance, would have become the dominant representation of Christ.
However, there were and still are modern translations of the Bible (beginning in the mid twentieth century) whose authors purposefully used illustrations that reflected the Semitic ethnicity and appearance of Jesus and his followers.