There are a few others as well. Is it a copyright thing? Were there guest stars from whom CBS didn't or couldn't get a release? Did they not release all episodes to give people an incentive people to buy the full seasons on DVD or on Prime.

I'm wondering if it's the music clearance problem that so dominated DVD releases for years.

And CBS is notoriously bad about getting these clearances, IMO (but they did clear it all for Lucy DVDs, though). They may not have been willing to pay again for streaming.