Heard about the 13th zodiac sign, Ophiuchus? (OH-FEW-CUS)
The sun does pass through that constellation from late November to mid-December.
Here's the simple reason: about 3,000 years ago, the Babylonians designed the zodiac. They needed 12 signs to match their 12 month calendar. The constellation Ophiuchus didn't make it. Astrology has used that same 12 sign system ever since.
If the Babylonians had picked a different system, the majority of Sagittarians would have been born under the 'Serpent Bearer.'