This is a good point.
There seems to be some kind of agreement between Mill Creek, Shout Factory and Sony or Universal for some previously released stuff. If I had to guess from looking at previous releases, Shout Factory might have "the right of first refusal" for some tv titles from...
Mill Creek has already showed their hand on this. Season 1 of Archie Bunker's Place has already been re-released by Mill Creek back in 2015.
Nothing since, which is likely a vote of no confidence in further seasons being release on dvd by Mill Creek.