This is the old problem of asking people who are still here, why people are not here. 
So as someone who follows the forum and even answers (and asks) the questions now and then, I can only offer my general impressions which may or may not be correct. However, it’s a point for starting a discussion.
First, I suspect that the guidelines post ( Please Read This First: Guidelines and Suggestions for posting LAMMPS questions ) may discourage some people from posting. It is very long and requires some time to read it. Especially for people whose first language is not English, it may be an additional effort, compared to just asking some LLM for an advice (I don’t use them anymore myself, so I have not idea whether they got better at LAMMPS - they were pretty mediocre before).
The guidelines require that the poster include a lot of info and some people may be leery of sharing their input script, either because they fear of someone “stealing” their research or that they will be criticized about its quality. Again, sharing the files with LLM may feel more safe. LLM won’t judge and most people don’t know that everything they share with it may be used in training the next generation (except some cases where the university or employer signed NDA with the LLM company).
Don’t get me wrong, I think the guidelines post was necessary and it did improve the questions that are asked, but it may also discourage some people from posting.
Second, the LLMs. As I said, I don’t use them anymore, so I can only suspect that they got better at answering LAMMPS questions. And even if not, I believe they may still confidently give wrong answers.
I think these may be two most important factors for the decline, but I’d like to be proven wrong as I don’t think there are any ways of fixing these two issues.