This is only a guess, but I'm assuming it's it's done that way to keep the intervals between each string the same. A bass player who plays a 5-string or a 4-string is used to only dealing with 5ths between each string. On a guitar, each string is tuned a 5th higher than the one above it until the B string which is tuned a 4th higher than the D string. This might throw off bass players who are transitioning from a 4 or 5-string, especially when most (not all) bass players lean heavily on 5ths and octaves.
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