It is entirely possible that the preserving of their dead meant that they simply weren’t afraid of death.
As opposed to our era of Christianity in which we cringe in terror at skeletons, witches, demons, and evil.
O death where is your victory! O grave where is your sting!
Those triumphant words of the Apostle Paul are the opposite of our cringing generation of Christians.
It's also entirely possible that preserving their dead is a phenomenal waste of time and an exercise in futility.
And it's also possible that the time, money, and other resources spent on preserving their dead would be far better spend on the living to demonstrate the love of Christ to a lost and dying world.
It's entirely possible the practice was taken from the occult and Babylonian paganism as much of Catholicism is.