Maybe when the Lizzies lobotomized our ancient DNA, we lost our significantly enhanced mental capacity to remember as well? In the Garden of Eden maybe we didn't even had to re-incarnate. "We" may have been bi-density spirits: while having a blast of driving & enjoying human bodies, maybe "we" could still fully remember our spirit Group Soul condition as well. That we could go home any time. Except the Lizzies didn't want to lose such a good energy source, so they lobotomized our DNA. Sheared our chicken-wings, so we lost the ability to fly back to 4thD-6thD.
Reason may be as simple, as how the C's said "the migrants are for votes". Any powerful personal computer today needs most of its resources to successfully emulate a decade old gaming console. Uses most of CPU resources. Our computer (just as our soul) cannot do much of anything else, because no more energy. Same way, I think, our souls are emulating our physicality illusion in a very convincing way and its supremely exhausting.
I call our 3rdD world "hard density", because its not malleable, like the 4th Density. Almost all fuel our souls have are used up to emulate our 3D physical world illusion. If God decided that he would add the burden of remembering incarnations on top of all of our Daily Life, I think, our souls would simply not have energy to run / power / fuel our 3D physicality emulation. We would panic, faint and go into coma or die really early, because of repeated "nerve shocks" if we would suddenly remember our often horrible earlier incarnations too.
C's said, 4th Density is easier to live in. I think it consumes less fuel to emulate the physicality illusion there. As 4thD is variable, malleable: thus 4thD doesn't exhaust the soul as much as 3D Earth. Therefore in 4thD our souls have surplus energy left to also remember previous incarnations.
5thD is even less physical than 4thD, so 5thD can powerfully serve as 'Reincarnation Review & Remembrance Central'.
I think the concept of "Hell" was invented in 5thD, by souls, who started to remember their previous horrible incarnations and began suffering intensely. It may have felt like burning. Thus the concept "Burning in Hell" was born. I definitely don't want to remember ever again my Cathar incarnation, where I was tortured to death by the amazing Spanish Inquisition. They had very little humor..
I don't watch Spanish movies, because listening to the voice of rapid-fire male Spanish speakers - causes first a slowly welling-up of uneasiness, which grows into fear. Then like sea waves, the past-life PTSD becomes stronger, until crescendoing in full-bore panic.
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So much is happening on all fronts and some things are a bit frightening, but all this work and effort prepared us, each in his own way, for it.. A huge thank you to everyone who participated 
