The most obvious reason is that pg writes an essay when he has an idea or an insight he wants to think and write about. New insights don't arrive on a fixed schedule.
If pg had been posting every month the HN thread would read: "Ask HN: is it just me or has the quality of Paul Graham's writing gone down?"
If pg had been posting every month the HN thread would read: "Ask HN: is it just me or has the quality of Paul Graham's writing gone down?"