More early 19th century accounts of the will-o-the-wisp, courtesy of the Worcestershire folklorist Jabez Allies, in
one of his books. These are mostly from around Alfrick, in the Malvern Hills. Allies later used one of these reports to track down the ignis fatuus himself, at Powick (see earlier in the thread).
What seems notable is that most of his informants describe seeing the phenomenon only a few times in their lives, which to me suggests the importance of particular weather conditions in producing it. This might be one reason it's rarely seen now: even if you lived next to a marsh with the right conditions present, it might only appear on a few nights a year.
Having read through more recent papers on the subject, however, I think the other main reason is probably down to the requirement for certain bacteria to be present in producing the necessary chemistry, specifically bacteria that originate in the human digestive tract. Two of the most recent reports from England (Mill Hill and Wherwell) are associated with sewage and with an old churchyard. I suspect that the near disappearance of the will-o-the-wisp is not just because marshes have been drained and improved, but because in general people are no longer sh*tting in (or at least emptying their chamber pots in) ponds, streams, and marshes.