Here's why:
* Charro refers to a type of Mexican horseman, specifically a skilled rider with a particular style of dress and riding. It's not a general term for a horseman or a person who rides horses.
* Antonyms are words with opposite meanings. There isn't another type of horseman or rider who is the direct opposite of a charro.
Instead of an antonym, you could think about related terms:
* Cowboy: A similar type of horseman, but from the American West.
* Gaucho: A South American horseman, similar to a charro.
* Urban dweller: A person who lives in a city and doesn't ride horses.
* Non-rider: Someone who doesn't ride horses at all.
Ultimately, the best "opposite" of charro depends on the specific context.