“If “L’Allegro,” which was created in Brussels in 1988 and concluded its fifth New York run since 1990 at the New York State Theater on Saturday, is Mr. Morris’s masterpiece, what’s he done since? Should we, as dance lovers and Morris admirers, be concerned that a choreographer still in his prime – he’s just shy of 49 – and celebrating the 25th anniversary of his company has not produced a comparable triumph in the last 18 years? And if not, why not? Here is a list of possible reasons…”