There is no explanation about the process outside of what occurs in the game. Unfortunately, there are some plot holes in the story, leaving it open to interpretation.
What we do know, is that the power of Meteor, or the Black Materia was "given" to the Cetra along with the White Materia, back when they were more involved in the planet's lifecycle. However, since they clearly valued the life of the planet over anything else, they decided the power of meteor should be locked away - never to be used.
Long ago, the Cetra were given two gifts from the Planet: the Black Materia, which had the power to destroy, and the White Materia, which had the power to protect. Being peaceful people, the Cetra embraced the power of the White Materia and rejected the Black, sealing it away by transforming it into a complex, booby-trapped, labyrinthine temple that became known as the Temple of the Ancients.
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The one thing that the game repeats is that the Lifestream is alive - it is the fundamental force that controls life on the planet. When something dies, that force is consumed by the planet, to be put into new life elsewhere on the planet. The other thing we know is that Materia is effectively "solidified knowledge" as it were
“The knowledge and wisdom of the Ancients is held in the materia. Anyone with this knowledge can freely use the powers of the Land and the Planet. That knowledge interacts between ourselves and the planet calling up magic... or so they say.” — Sephiroth
This means, that the planet, when it created the Black and the White Materia, this was the manifestation of the planet's most powerful and most precious knowledge - effectively, life and death.
Now, the issue here is that there is no solid timeline as to when this occurred, how long the Cetra had been on the Planet, and when Jenova Arrived. The only discerning facts we have are that Jenova arrived 2000 years before the events of Final Fantasy 7.
So, there is nothing to say that the creation of the Temple of the Ancients is actually linked to Jenova. This is where the theories float around. It could be that the Temple was simply a defense against the Cetra - a people that coveted life so much that the thought of sacrificing oneself in return for the power to create such wanton death was the ultimate price. Or it could be that the idea of someone so radical would be the only one willing to do so, and therefore the plan would never eventuate. Amongst a people so devout to the cycle of life, the idea that any more than one person would be willing to carry this out was beyond their realm of understanding.
The issue with this logic however, is that it does not account for the possibility that others may follow. This is kind of explained, in that the Cetra were such a peaceful race that conflict was such a foreign concept to them, that the manipulative tactics that Jenova used were just too effective - able to wipe them all out to near extinction.
So, following this logic, the plan behind the Temple of the Ancients would have appeared as foolproof to them, to the point where no one would be willing to sacrifice themselves, and therefore, the temple would remain standing forevermore; no failsafe required.
So, in summary; there is no explanation for this, simply because the gaps in the lore do not allow for one. vague lines can be drawn between certain points in the lore, but without more information, there is no way to clarify this.