Press, speech, religion, assembly, while loosely related it seems they could have all had their own amendment. Compared to other amendments like two, and five that seem very specific.
All of those categories really do come down to "free speech" in the end anyways.
Ignoring that, there's no rules that dictate what can and can't be in one amendment. There's a lot of extremely specific amendments (Prohibition comes to mind, as an example), and there's some that are much broader, like the First Amendment, which covers general speech/information issues, or the Fifth Amendment, which broadly covers criminal trials, due process, etc.