Memory about the past life As per the scriptures, lower being such as pretha &amp; higher beings such as deva &amp; Brahma remembers their early life. But in case of humans, only rare incidents are reported in remembering the past life. What is the cause of this?
This is a limitation of we who inhabit a three-dimensional world IMHO. We are not aware of other dimensions, and do not have the ability to see past existences because of the limitations of our physical senses. We experience only a very small part of reality, the three-dimensional world of physical forms.  But also we could quite rightly say that we humans live in a four dimensional world; in other words, humans are aware of four dimensions -three dimensions of space plus time.

This is why we see only two realms (human and animal) out of the other realms of existence like that of Pretha, Deva and Brahma. Of the newest scientific theories, the M-theory says that ours is a 11-dimensional universe, even though we can see only four dimensions (including time). So Prethas live amongst us, but they live in a differnet dimensional world.

Our consciousness (vinnana) is not like that of an Arahant. Ours is contaminated by defilements. This our awareness is not pure; it is like looking though a foggy window. But it is not so for some Arahant who got to open the divine eye in them. Twenty five hundred years ago, the Buddha achieved the ultimate state of the mind and became “all knowing” (or “perfectly conscious”). He came to know about the boundless universe, the endless cycle of rebirths, and about the complex process of cause and effect (paticca samuppada) which sustains everything in the universe.