Here's why:
* Common nouns refer to general things or concepts. They are not capitalized unless they are the first word of a sentence.
* Proper nouns refer to specific people, places, or things and are always capitalized.
While the equator is a specific line on the Earth, it is not a unique name like a city or a person. It's a general term that describes the imaginary line that divides the Earth into the Northern and Southern hemispheres.