I was never satisfied with the explanation that the turaga didn’t tell the matoran about Metru Nui because they didn’t want them to yearn for a home they might never see and couldn’t remember anyway. Especially how secretive the turaga became about that knowledge.
Why shouldn’t they have at least tried to tell them?
Because I think they did. Or tried at least in the very first year on Mata Nui.
Unfortunately, no matter how knowledge imparted - everything that related to the city, the matoran could never retain. Aside the complete erasure of their past memories, the spheres had also inflicted temporary anterograde amnesia to everything about Metru Nui. (It was likely another feature Makuta had programmed into the spheres.)
The turaga would tell a story about the vahki- when the same topic came up later just a day later, the matoran who had heard the story would ask what these ‘vahki’ were. They were simply unable to retain the information. Worse, if the matoran tried to recall what they had been told, some would suffer severe headaches.
With these repeated failures, and danger to the matora’s wellbeing, the turaga soon gave up to tell the truth. Besides, it slapped them every time when they were reminded the matoran had forgotten, and the six of them were alone in knowing what they had lost.
They went instead with a mythical origin story to create identity for their new society. This is also why Vakama adapted some details about Lhikan’s life so there were no direct links anymore to Metru Nui.
These events also led, indirectly at least, to the turaga’s secrecy.
They already had been told they weren’t their home’s destined toa; then it was revealed to be a lie. This new experience did the rest to skewer and distort their perceptions, just because it hurt them so much. It seemed that the truth only caused trouble and misery (including for themselves), and didn’t seem to really matter. They unconsciously began to hide more and more things until everyone but Nokama came to an unspoken, unknown agreement that nobody but them could handle the truth. They were unable to see the fallacy in their reason, even after the spheres’ secondary effects wore off and the matoran showed that they could hold onto new information about their past.
Only Nokama refused to believe this policy, as she had realized what keeping secrets did. But in at least two point she was like her siblings.
For one, they were all surprised that the matoran of Mata Nui kept talking about their tales of Metru Nui after their first story session. And that the world hadn’t ended.
Because even more importantly, none of the turaga realized that each of their siblings were as terrified of the consequences of the truth as themselves.
