My impression is that a bunch of it has to do with "4K UHD" Blu-Ray uses just about all the space for the main feature and required languages per region and has very little spare room. (As did "3D" Blu-Ray, though those are mostly extinct today.) The early solution to that was "4K UHD" releases were all expensive "Collector's Editions" that bundled a standard Blu-Ray and/or DVD with all the "special features" including commentaries (increasing the marginal costs of all the packaging by including multiple discs). The late solution today seems to be to just ignore all the "special features" entirely, planning only for the lowest common denominator amount of extra space, and things like commentaries are slowly disappearing as an expected bonus feature (again; it's not like these features existed in VHS or Laserdisc; DVD was its own weird golden age).
Laserdiscs had extra audio tracks with commentary on some discs as I recall. (But the stack of LDs I have is in the garage and it's too cold to check :-)) An audio track shouldn't take much room. But not sure about actual numbers.
Huh, all the normal non-collectors UHD releases I got so far still come with a HD Blu-Ray. If that has changed it must be recent enough that I haven't run into it yet.
