Classical has several centuries of music, ranging from modern classics to chamber music, baroque, piano, string quartet and full orchestra compositions to name a few. And there is a range of styles from Respighi to Ravel without even touching Beethoven, Mozart, Tchaikovsky and the big name composers. Someone wrote that you could listen to different pieces of classical music for several months and never hear a repeat. The K index for Mozart alone has over 200 pieces.
At the first station I worked for, back in the mid-1970s, we had a four hour Classical program every night, and my original shift included being the board op/producer of the Saturday edition (the station owner voicetracked it). I later became the regular weeknight producer (and fill-in host) and my recollection was that the "major" works were repeated no more often than once per three months, with each night's program being at least one-third lesser works. We also used a different orchestra's rendition of the major pieces when we repeated. I don't remember our audience complaining about repetition and I don't remember being bored from listening to that music four hours a night, five nights a week.
I still have a copy of Grofe's "Grand Canyon Suite" by the New York Philharmonic that I pull out and play once in a while. My quirkiest affection from that era was Seiji Ozawa conducting the San Francisco Symphony, with the Siegel-Schwall Band, in a performance of William Russo's "Three Pieces for Blues Band and Orchestra", which we played about once a year. The audience didn't even complain about that, even though it borders on progressive rock.
But, of course, Smooth Jazz stations did not go three months to a year before repeating, and after a while it all either started to sound the same, or at least not varied enough to hold an audience's interest for long periods of time. Or, for some, hearing the same songs over and over did drive them away after the 24th playing of a Strunz & Farah or Ottmar Liebert track in a given week.
I think, as was said elsewhere in this thread, that SJ was seen as the inheritor of the BM audience ... at least to the radio stations that embraced it. But ultimately, it lost a lot of the audience that originally listened out of curiosity and became unsustainable as a format, except for a handful of markets where creative sales management continues to keep it viable.
Just as the Oldies format is becoming more and more relegated to lesser signals where it survives, the same will happen to Smooth Jazz. Let's face it, KOAZ, which is (rightfully) a favorite among SJ aficionados, is still on an AM station plus a 250-watt translator, and eventually that will probably go away too, just as Oldies, and Beautiful Music, and MOR, and ...