I listen to classical music, jazz, and 60’s to 90’s rock, and appreciate works from all of those genres. There is a huge variety of classical music out there, from operatic things to Vivaldi to Stravinski to Bach to Liszt. There’s not much it all has in common, really. There are some really cool but very short classical pieces, under 30 seconds in length, and there are some very long, many-hour symphonic things that you need an entire evening to take in.
But I think there are a few things that hurt classical music as a “popular” format these days. One is that generally speaking, rock is more “instantly gratifying”. There’s a very clear thumping beat, there are almost no dynamics (it is all compressed to one volume level in the mastering process), and it tends to be short and musically very simple.
Of course anything you say about a genre of music is by nature a generalization. But in contrast, much classical music has an enourmous dynamic range (not so good for playing over the radio), large variations in tempo, themes that unfold over a longer period of time (not so good for today’s 60 second attention span), and often classical pieces spend much more time before the final “resolution” of the tension that’s produced by straying from the song’s resolution key. Also, it has very soft, sedate passages which are meant to be heard in contrast to loud, furious ones, but who these days will sit through 15 minutes of soft, slow music? It takes more effort to appreciate the whole of a large work, than to appreciate a small standalone rock song that’s never more than 5 or 10 seconds from resolving to a base chord.
Also generally speaking, it requires much more musical talent to play classical music than rock. For instance, I am an amateur pianist (about 19 years experience), and while there are certainly talented rock pianists out there, a very large fraction of rock piano is just pounding out repeated chords. By contrast, many of the works of Liszt, Chopin, and others are enourmously harder to play, so this tends to limit the interest among novice musicians. Almost anybody can learn to bang out a rendition of Stairway to Heaven, but only a handful of people in the world can really play some of Chopin’s harder piano compositions.
All told, I think classical music, just very generally speaking, is less accessable than rock, and thus cannot be as popular. I don’t see that as either good or bad; it just is. I surely spend more time listening to rock than to classical - i guess perhaps a 4 to 1 ratio. But probably my top 5 favorite works (in all genres combined) are all classical pieces. (Many of my least favorite works are also classical pieces).