HOUSTON — Why hasn’t Mark Morris set more dances to works by Joseph Haydn? An important part of this famously musical choreographer’s talent has been applied to pieces by composers from the late 17th century to the late 18th — notably Purcell, Bach, Handel, Gluck and Mozart. Haydn (1732-1809) isn’t just a central composer of that period; he also often exemplifies the same qualities as Mr. Morris: high spirits, terrific humor, a strong inclination to the pastoral and a keen instinct for structural experimentation. Last week, the Houston Ballet gave the premiere of Mr. Morris’s “The Letter V” — choreographed to Haydn’s Symphony No. 88 in G (1787). (Some Haydn symphonies were once known by letters of the alphabet; V was assigned to this one — hence this ballet’s title.)