Why is the New York City Ballet the greatest ballet company on the face of the earth? Glance at the agenda the troupe is bringing to the Kennedy Center Opera House for a three-week run starting Tuesday night --

To start with, there'll be u9 ballets, all but one of which will be packed into the first two weeks of programming, since the final seven performances will be devoted to the evening-length "A Midsummer Night's Dream." They range from masterworks, like "Four Temperaments" and "Goldberg Variations," to such adroit entertainments as "Stars and Stripes" and "The Concert." The sheer profusion of them would be impressive enough, but it happens also that their breadth is equaled by their depth.

What we'll see at Kennedy Center will be the best assortment the company could muster, from an active repertory of 75 ballets, in the after-math of a debilitating five-week strike in New York by orchestra musicians. Now, assuredly there are other major ballet troupes in the world with varied and extensive repertoires. But the 19 works the New York City Ballet is presenting here, including three Washington premieres, are entirely the products of two men: George Balanchine and Jerome Robbins. For this there is no parallel among other companies, here or abroad, in this century or in any other.

Balanchine, at 73 the company's choreographic mainspring for more than 40 years, is in himself a veritable compendium of balletic idioms and approaches.

"Union Jack," for instance -- Balanchine's hour-long "Bicentennial" extravaganza which will be seen here for the first time during opening night at Kennedy Center -- draws its imagery and overall shape from the marches and musical hall balladry of Great Britain.

"Square Dance," Balanchine's new version of an opus from 1957, finds formal, classical underpinnings for rustic Americana, and "Stars and Stripes," his mammoth ballyhoo spectacle of 1958, does the same thing for John Philip Sousa. In the elegant "Chaconne" to the music of Gluck, Balanchine's springboard was the poised geometry of an idealized antiquity, while in "Le Tombeau de Couperin," from the Ravel festival of 1975, it was the ornate decorum of the French rococo.

"Meditation" and "Tchaikovsky Pas de Deux" are Russian schmaltz, mediated through Balanchine's own brisk modernity. "Bugaku" takes its garb and manners from the ritual eroticism of Japan. "Serenade," dating from 1934 but ever youthful, and the dazzlingly complex "Four Temperaments" are both quintessential Balanchinian abstractions, making dancers' bodies into the visible trace of a musical impulse.

This still leaves the highly contrasting and almost unclassifiable --because they are such thoroughly individual dramatic conceptions -- "La Valse," to Ravel's phantasmagorical score, and "A Midsummer Night's Dream," the beguiling, funny and lyrical reverie Balanchine concocted upon the framework bequeathed to him by Shakespeare and Mendelssohn.

All this is still only a sampling of the Balanchine treasury -- not a single one of his landmark settings of Stravinsky is included, for example. If the gamut suggests an awesome catholicity of temperament, one has only to look at Balanchine's past to see it confirmed in his artistic evolution. Once he settled in this country, after a youth in imperial Russia and a brilliant Parisian journeymanship with Diaghilev, nothing in the whole spectrum of American show business remained exempt from his curiosity or creative meddling. Broadway musicals, Hollywood movies, the opera, television, even the circus (he and Stravinsky actually did an elephant ballet for Ringling Brothers in 1942) --Balanchine went.

And even Balanchine, of course, isn't the whole of the New York City Ballet story. There are the dancers, without whose power and flexibility such a repertory would clearly be impossible. Suzanne Farrell, Patricia McBride, Allegra Kent, Karin von Aroldingen, Sara Leland, Patricia Neary, Jacques d'Amboise, Peter Martins, Helgi Tomasson, Jaen-Pierre Bonnefous, to name some of the most noteworthy we'll be seeing this time --only in a company headed by Balanchine would such dancers rate second mention after their ballet master.

And there's Jerome Robbins, who's a whole theatrical epic unto himself, and whose work, miraculously enough, far from being eclipsed by Balanchine's proximity, seems enhanced by the linkage. Besides the Washington premiere of "Other Dances," which Robbins created last year for Natalia Makarova and Mikhail Baryshnikov, and the aforementioned "Goldberg Variations" and "The Concert," Robbins also will be represented by his "Dances at a Gathering," "Fanfare," "Afternoon of a Faun" and "The Cage." Even in this dance-mad era, a ballet bonanza of this dimension doesn't show up hereabouts very often.