Why were the forms of the gismu (root words) created from several languages, rather than just one, for instance English?
Because of the ideal of cultural neutrality, one wanted to make the learnability of the vocabulary the same, regardless of one’s native language. Also, making the vocabulary of Lojban too similar to a single language (English has been proposed many times, because it is widely used) would make learners who were familiar with that language import semantic and cultural baggage from that language — what in linguistics terms is called a substrate language.