Because endings for adjectives and nouns should match.
From the Tips and Notes for the Accusative skill:
Note that an adjective modifying a direct object would also receive the -n ending. Thus, Virino kisas belan viron. would mean "A woman kisses a handsome man." As explained above, we could also phrase this sentence as Belan viron kisas virino, which would still mean the same thing with more emphasis on the good-looking man.