Short answer: It was a decision by the american publisher. The European edition always had the final chapter.
The book has three parts, each with seven chapters. Burgess has stated
that the total of 21 chapters was an intentional nod to the age of 21
being recognised as a milestone in human maturation. The 21st chapter
was omitted from the editions published in the United States prior to
1986. In the introduction to the updated American text (these newer editions include the missing 21st chapter), Burgess explains that when
he first brought the book to an American publisher, he was told that
U.S. audiences would never go for the final chapter, in which Alex
sees the error of his ways, decides he has lost all energy for and
thrill from violence and resolves to turn his life around (a
slow-ripening but classic moment of metanoia—the moment at which one's
protagonist realises that everything he thought he knew was wrong).
At the American publisher's insistence, Burgess allowed their editors
to cut the redeeming final chapter from the U.S. version, so that the
tale would end on a darker note, with Alex succumbing to his violent,
reckless nature—an ending which the publisher insisted would be 'more
realistic' and appealing to a U.S. audience. The film adaptation,
directed by Stanley Kubrick, is based on the American edition of the
book (which Burgess considered to be "badly flawed"). Kubrick called
Chapter 21 "an extra chapter" and claimed that he had not read the
original version until he had virtually finished the screenplay, and
that he had never given serious consideration to using it. In
Kubrick's opinion, the final chapter was unconvincing and inconsistent
with the book.