posted September 04, 2015 07:04 AM
I suspect the reason is an astronomical one from the time our astrological system was shaped (which was mostly in the middle ages). Sun in the centre relating to Leo
Moon/Earth-complex to Cancer.
Those are apparently seen as one system, even though we all know of course, that Sun is pretty far from earth.
anyway the closest planet to Sun is Mercury, hence he is given significatorship over the signs that flank the sun and moon based signs, so Mercury and Virgo.
Then moving on from Mercury we bump into Venus next, so Taurus and Libra the next signs, are being governed by Venus, after that we pass Mars, - and Scorpio and Aries.
After that we stumble into Jupiter, Sagittarius and Pisces and conclude with the Saturn-ruled signs Aquarius and Capricorn, which are of course opposed to Leo and Cancer.
there might be other reasons, too, of course.
Anyway, when Uranus was being discovered the nice structure was being broken up of course, and more and more objects are being discovered, so it is questionable to some astrologers to attribute any kind of significatorship to them.
Personally I think Sagittarius is described pretty well by Jupiters significatorship. Including the shadow sides of Jupiter as portrayed in mythology.