Supreme Law Rules

The primary reason was the Primordial Stone.

With an ample supply of Primordial Stones, during the crucial moments of tribulation, stabilizing the Universe Origin Source with these stones would ensure a successful passage through the tribulation.

Failure in tribulation would lead to the universe's ruin.

This was because tribulations targeted the universe condensed by cultivators, not the cultivators themselves.

The universe was meant to be the highest law rule, the origin of all things.

Cultivators, by condensing the universe, were defying this natural order.

Hence the tribulations.

Tribulations targeted the universe; dispersed tribulations couldn't condense the power of Life and were also targeted by the highest law rule.

It was a form of minor punishment.

After achieving perfection in the Dao Heart, the Emperor was the best endpoint tailor-made by the highest rule.