It's the platforms the speech is carried over.
Are you broadcasting a message over television? The stations have strong control over the content of your message, and will charge you for the privilege. This actually meant that vulgar content generally stayed out of mainstream television.
Is the message distributed online, over a social media platform? These companies don't care about the content as much because it's harder (more expensive) to police content generated by millions of users as opposed to a handful of advertisers. They'll care to the extend of the law, but now your seeing more vulgar content of the lowest common denominator, placed along side some of the best content ever made.
Are you shouting in your park? Well, your local municipality might not want you shouting vulgar things in the presence of children and kids.
There also seems to be a psychological aspect where people are drawn to outrage and explicit content, which fuels the click-driven algorithms, and the news networks that rely on invested viewers.
One of the best parts of the internet, in my opinion, is Wikipedia. It's a user-driven, content-focused website that doesn't sell to advertisers. It's not perfect, but it shows that a good platform with clear and carefully chosen rules and it's own source of funding can actually be a great benefit to humanity.