Because it is very often used towards gay men. Not exclusively, but often. It's regional, too. Not all Spanish speaking countries use it that way, and some use it that way but with a more general connotation. You'll get a lot of people defending it based on tradition, or based on how THEY use the word (ignoring how others use it), or based on it having many meanings, or based on racism towards their culture. They'll complain that this is white people who don't speak Spanish telling them what is and isn't offensive in Mexico.

But ask a gay man in Mexico what it means to him when he hears "puto" and it won't be about his mother.