The fact is that the majority of topics have been done to death and the postings get distilled down to the same old subjects with the same people pontificating on them.
When anyone has a different opinion they are generally pounced on and beaten into submission, anyone who stands against them then has the usual whispering campaigns etc started against them.
And there are a couple of posters legendary for behind the scenes antics of trying to discredit those they have a grudge against. It is is boring and off putting for many people to be involved in that sort of behaviour and so they drift off onto other things.