I still don't feel like this article gets to the bottom of this mystery.

One answer I saw might be that the tray mechanisms were too big to fit in a 5.25" bay. The early Mitsumis used a manual tray, but I always assumed this was for cost reasons.

My Philips burner from 1992 was tray loading.

https://www.computinghistory.org.uk/det/44095/Philips-CDD-52.../