This is a complicated topic with a lot of nuances to it if you think about it thoroughly.
I totally get that people want to play ALL of the new TWW dungeons on M0 difficulty (me included). It’s new, it’s exciting and we want to experience those dungeons.
However I can see why they did not do this for a combination of these reasons:
- We got the M+ balancing revamp, so that M0s should roughly equal previous +10s. What they wanted to achieve by this is that people can actually learn all mechanics properly in M0, without just facerolling through them like in previous expansions (whether this worked or not is another topic).
So M0 is supposed to be the “proving ground” for the upcoming M+ dungeons.
However, if we just had those 8 TWW dungeons on M0 now, nobody could get a hang of the 4 old dungeons on the new difficulty. This kind of forced them to include the 4 old dungeons to be available as M0s.
So why not just have all 8 TTW dungeons AND the 4 old ones as M0? Well, this brings us the the second point.
- If they did that, we would have 12 M0 dungeons on a daily reset. This would make a lot of players feel forced to complete 12 dungeons a day in order to stay competitive for the upcoming M+ start.
12 dungeons per day can take a long time. A few hours at least for most people. And that on a daily basis.
Ofc you could now argue that “Well it’s not our fault some players feel this way… they don’t HAVE to do that” and you’d be correct but on the flip side you also have to keep in mind why the so heavily discussed timegating exists in the first place.
A lot of people argued that timegating was necessary because they would feel forced to spam M+ asap, so they don’t fall behind. Timegating was welcomed because it made them be able to appreciate the story, side quests and other activities before the season starts.
The combination of all of those facts pretty much is, probably, or at least in my opinion, the reason why Blizzard opted to just include the 8 seasonal dungeons in the M0 difficulty, rather than just the 8 TWW dungeons or all 12 dungeons.
TLDR;
Having seasonal dungeons on M0 was necessary for “training” purposes so people can experience and play the seasonal dungeons on M0 difficulty before M+ opens.
All 12 dungeons probably were not released because it would’ve overburdened a lot of people to farm 12 dungeons per day.