The Devs apparently learned very little from the bad aspects of Eureka.

[Amnmaat]

...........why not just have all 72 have a go at it...........

It's not realistic to expect all 72 to be present or available. Not even in eureka with 100+ players was it realistic that every instance would reach 100+ or that all 100+ would be there for every NM.
If you allow all 72 you'd have to balance it for all 72 or they become what we saw in Eureka with NMs, a boring zerg without well thought out mechanics.

They're learning from watching World of warcraft raids, it's very evident from what I see of Bozja, which is a great thing. What they need to do in my opinion is to have multiple critical missions happening at the same time so people still get a chance to do something; but even if they don't I prefer this to the AFK-Eureka.