Why did the author include the detail at the end of 'Flowers for Algernon'?

Those "details", including the devastatingly sad ending, are a part of the drama of the book's history. The original publishing house offered the short story rejected it as they wanted a "happy ending". The story the way the author wanted went to a different publishing house, and the story won the Hugo that year.

(The Hugo is the scifi author's equivalent of an Oscar in prestige.)

Then the author offered the novel version to Doubleday, and they turned it down because of the sad ending, and were insisting on a happy ending. The author remained firm, took it Harcourt, and they published it with those details and sad ending. It was a runaway bestseller, and has never been out of print since, though it's been nearly half a century!

(And the novel won a Nebula award, the scifi author's version of a Golden Globe in prestige!)