This is actually largely to do with what metadata they take and then how they present it. And it’s really tricky.
The metadata associated with classical music is generally speaking more complex than (say) a rock track. Until relatively recently there was not (IIRC) a clear format laid out in the DDEX standards (which are the metadata standards associated with delivering music and other media) for how you present the numerous different elements of metadata associated with a classical release. So it is a real mess.
I don’t think Spotify necessarily even takes the full metadata spec that you can associate with a classical release.
So for example: you may have a recording of the Allegro - which is the third (also written III) movement from Beethoven’s Symphony number 6 - also know as the Pastoral, also known as Op.68 or Opus 68 (and as the “Pastorale” in German) which is in the key of F major.
I need to be sure that the track I’m listening to is not the 4th (IV) movement which is also Allegro and definitely not the 1st (I) which is Allegro but not too much - Allegro non troppo.
Now - I want the version recorded in Austria by the Weiner Philharmonic conducted by Simon Rattle produced by Stephen Jones and released on Warner Classics. You however may well prefer the Decca version conducted by Erich Kleiber with the London Philharmonic (but not the London Philharmonia!) produced by Victor Olof. Or you may want the Kleiber-conducted version with the Royal Concertgebouw Orchestra. How do you distinguish?
As you can see there are a whole load of different pieces of metadata on which people might search - and when it comes to classical repertoire these things really matter to the listener. The way labels try to address it currently on Spotify is to simply stuff as much as possible into the track title. Spotify now have some of this listed in the “song credits” but that is relatively recent. And the info on many tracks is far from complete.
There are some specialist classical services addressing this - but the problem is I would prefer not to have to subscribe to an additional service just for classical music when most of it is sort of available on my streaming service of choice.
So yeah. It’s a mess :)