The method prescribed in the Tyrant Bull Marrow Secrets wasn't really as efficient as at the start, one's capacity for Immortal Essence would be quite low.
Though at the latter stages, it was said that once one had reached the full sized totem, their capacity for Immortal Essence would exponentially increase too. Of course, reaching that was already a tall mountain to climb.
There was a higher chance that before one reached that, they would simply end up dying.
Then there was the fact that the Tyrant Bull Marrow Secrets was a body cultivation technique that needed one to regularly battle and fight. Without that one's cultivation base wouldn't increase past the second or third form of the totem.
It was a body cultivation technique that relied on the regularly stimulation of the body through combat and use of the Tyrant Bull Marrow Secrets. It required one to expend Immortal Essence in order to progress.
