" WHY DO WE USE THE MOON SIGN INSTEAD OF
SUN SIGN / ZODIAC SIGN "

Astrology Moon is the signifactor of the MIND. Since Moon is the HEAVENLY BODY CLOSEST TO THE EARTH, the magnetic influences of all other planets reach EARTH through the Moon.
Also since the mind is ruled by Moon and all influences on the HUMAN BEING whether PHYSICAL, MENTAL, PSYCHOLOGY OR SUPERNATURAL effect the HUMAN MIND. The HUMAN BODY which is ruled by SUN may or may not respond to the mental / psychological / supernatural influences.

THIS IS THE REASON THAT INDIAN ASTROLOGY RECKONS THE PLANETORY MOVEMENTS BASED ON YOUR MOON SIGN (RAASI).