We lived pre-mortally as body-less spirits in the presence of God and chose our mortal incarnate lives on earth - that is, we chose to be born with bodies, live where we live, and we chose to die.
So, why can't we remember our pre-mortal existence - and why are we out-of-contact with Heaven?
The short answer is that we are not: we do remember that we had pre-mortal existence, and contact with Heaven does happen.
But the memories are imprecise and the contact imperfect.
Why? Why are memories unclear, why is contact unclear?
Because of the necessary qualitative difference between earth and heaven, and between spiritual beings and incarnate beings; necessary difference, due to the function of earth and incarnate life and mortal death.
If we are to have a chance (a chance; not certainty) of spiritual progression beyond what was possible in out pre-mortal life, discarnate, spirit-life; then we must be incarnate, and we must live here (and therefore not in Heaven), and we must die; and all of these were and are choices - for incarnate mortal earthly life to 'work' then we must continue to choose it.
(Even death. If we do not willingly choose to die when it is our time to die - and we will know this time - then by that failure to accept death as both necessary and good, we will be rejecting our destiny.)
But earthly, mortal, incarnate life is qualitatively different from Heavenly, spiritual life (whether pre- or post-mortal); and to communicate across this difference is necessarily imperfect.
It is (analogously) like the imperfect communication between an adult and a three-year-old child; or trying to understand a related but different language (like a modern American person listening to Old English/Anglo Saxon); or like trying to understand a tape recording slowed down a hundred-fold.
So, time runs differently for us, space has a different quality, and in our lives there is noise and distraction, and our bad choices, to contend-with.
So there are memories and there is communication; but these are necessarily difficult.
Necessary not because the difficulties are in and of themselves necessary (clearly, God wants us to remember and to communicate - and his revelations are full of reminders and advice on communications such as prayer); but necessary because of the necessary nature and quality of this earthly, mortal, incarnate life - which we chose - and the necessary difficulties of which we accepted.