If that's the case, I think it's misleading. It's fine to fork a project, but you don't get to call yourself the next generation of someone else's project.
What improvements have been made to make them better? The problem domain seems pretty well defined and even 20 years ago the things that were changing felt like polishing off a few rough edges caused by earlier resource constraints.
I don't want to be dismissive and say "Why make this?" as a implied suggestion that it shouldn't have been made.
Nevertheless, Why make this? I assume there are good reasons for doing this that I am not aware of, what are they?
I dislike Java as much as the next guy, but I believe the true value of tools (and this tool in particular) is in the embedded wisdom and experience of their creators/Terrence Parr. Just generating a functionally equivalent port doesn't add much value.
That said, that's just a first impression, I have no idea what motivated this fork
I didn't really care about diagnostics. It has some, that's enough.
And of course it's easier to maintain declarative grammar description.
My guess is, that it's often used for those kinds of simple grammars without high requirements to impementation. When you need to get things done. Like regex. You might write code to parse a string in a more efficient way, but with regex it's almost always easier. So ANTLR is like regex engine for more complicated inputs.