I’m just guessing, but there’s a possibility that mechanical limitations are the reason. Adjacent strings’ mechanisms might have problems if their linkages and rods travel alongside each other throughout the mechanism. It’s an interesting question, I’ll bet this was well explored when the pedal harp was developed and a little reasearch will turn up the answer.
As I mention in the thread that has been linked here, the fact that they alternate by fifths seems a deliberate things to me. It makes modulating through the keys by alternating left to right make much more sense. Keys proceed by fifths, and the pedals on the harp alternate back and forth by fifths. I can’t imagine that was coincidence or wasn’t developed with practical application in mind.