Remembering past lives Why do we not remember our past lives?
If it is possible to recollect one's previous lives by deeply concentrated thought,is it not possible for us to visit the hermits living in Himalaya area and verify the truth of doctrine of rebirth?
Why do we not remember our past lives?

It is a protective mechanism, kind of like how when you enter REM sleep your body paralyzes you to avoid hurting yourself. Similarly most of our chakras are in a semi-open, unpurified state because the full power of it can be harmful to ourselves and others. Many spiritual powers remain dormant. (I would not rely on the explanations of famous modern teachers because most of them are just intellectuals and have not put the prerequisite solitude and focus to attain realization... and sometimes they have to make publicly acceptable comments to avoid attack.)

If it is possible to recollect one's previous lives by deeply concentrated thought?

Yes. By entering the 1st jhana, one can even recall one's past lives. The below passage from MN 36 indicates Buddha went to 4th jhana before doing so, recalling many more. I would refer to the book Mind Experiment by Bavo Lievens for references on Buddhist texts, as well as beautiful Buddhist cosmology and how to go about doing this. Go to page 80.

Here is an excerpt from MN 36:


  "When the mind was thus concentrated, purified, bright, unblemished,
  rid of defilement, pliant, malleable, steady, &amp; attained to
  imperturbability, I directed it to the knowledge of recollecting my
  past lives. I recollected my manifold past lives, i.e., one birth,
  two...five, ten...fifty, a hundred, a thousand, a hundred thousand,
  many eons of cosmic contraction, many eons of cosmic expansion, many
  eons of cosmic contraction &amp; expansion: 'There I had such a name,
  belonged to such a clan, had such an appearance. Such was my food,
  such my experience of pleasure &amp; pain, such the end of my life.
  Passing away from that state, I re-arose there. There too I had such a
  name, belonged to such a clan, had such an appearance. Such was my
  food, such my experience of pleasure &amp; pain, such the end of my life.
  Passing away from that state, I re-arose here.' Thus I remembered my
  manifold past lives in their modes &amp; details."