I think it is something like this.
When MusicBrainz was first created it was focussed on Pop/Rock. Peoples minor interest in Classical was very much focussed on the composer of the music, so being being able to list all tracks attributed to a composer was deemed more useful than all tracks attributed to a performer or conductor. They couldn’t do both because Advanced Relationships did not exist. Artist Credits allowing multiple artists to be credited and Recordings as a separate entity didn’t exist either, also meaning that Recording Artist didn’t exist.
Its very difficult to change the way things work after the event, but it doesn’t matter that much since TrackArtist is sort of a meta- meta field, the composer can usually be found by looking for compose relationships on the recording, the conductor for looking for conductor relationship and so on.