Because Orion is not on the path of the ecliptic, which is the Sun's apparent motion through the constellations. The zodiac signs are named after constellations on the ecliptic. read more
The constellations of the Zodiac are the ones that, from the perspective of the earth, the sun moves “through” with the seasons. The constellations of the zodiac are located on a band called the “ecliptic.” read more
It's very prominent in the night sky and the 3 belt stars make it easy to spot. You'd think that, when the Zodiac was being made, Orion would have been one of the 12 signs. read more
Why the Constellation of Ophiuchus was Not Included in the Zodiac By Mark Lerner Back in January of this year, an astronomer and professor in Minnesota (Parke Kunkle*) gave an interview to a reporter at the Star-Tribune newspaper in which he tried to disparage Western Astrology. read more
It is a matter of common knowledge among mystics that the evolutionary career of mankind is indissolubly bound up with the divine hierarchies, who rule the planets and the signs of the Zodiac, and that the passage of the Sun and the planets through the twelve signs of the Zodiac, marks man's progress in time and in space.Source: quotemaster.org