Could it be that he never checked whether all the hints he gave sufficiently narrow down the location to just one spot? That he has additional knowledge for every line, knowledge which he thinks is self-evident, but knowledge with is actually just very personal to him, and cannot be inferred?

In this case, the riddle might never be solved, and he might die in the self delusion that all the attention this raised is because he is a gifted writer, when the reality is much more sad: he was a terrible riddle author that threw away a million bucks.