Why only Scorpio has two main stars?

Because when the three kings were not discovered, astrology had risen. Except the sun and the moon, the other five stars control the two constellations respectively.

The ruler of Scorpio is Pluto. In the more ancient traditional astrology, the ruler of Scorpio is Mars.

Astronomers are worried about Pluto's abdication, and astrologers are also in an awkward position. When Pluto leaves, Scorpio is likely to be dominated by its old star Mars again, and Scorpio's concept of the eighth house will also change.