Copywriters get paid more. 😉 Seriously, the term “copywriter” is older, and it refers primarily to people who write advertising and marketing copy. “Content” has become the catch-all term for material (not always text) that gets published on websites, and its purpose might or might not be to sell something. Blog posts are “content.” You might hire a “content writer” to write blog posts for you, but not a copywriter. (And me, I’d hire a blogger.) And you might not want to hire an all-purpose “content writer” to create a sales letter for you, either.
Writing advertising copy is a specific skill. So is technical writing. So is journalism. Very few people are equally good at all types of writing, so even though it can be convenient to lump all online writing under the heading “content,” it’s not very useful if you have a specific job for which you need to hire a writer.