Sorry for my ignorance but why is that all gods except Helm had to walk on earth as mortals, and why didn't helm walked amongst them.

Is that yesterday, when i finally reached the end of chapter 6 and was about to enter the elven city i decided to go to Watcher's Keep, the guy that give us the misson of reinforcing the seals that maintain the imprisoned one within the keep refered that, i already knew that the gods were forced to it and already read the Book of the dead three (Book that tells the Story of Bane, Bhaal (the assassin) and Myrkul (the necomancer)), but yet i don't know.

This is part of the story of the time of troubles where all Gods were made flesh and forced to walk the earth.
Helm was appointed as a guardin of the celestial stairway which lead back to the heavens.
The history of the Dead three is taking place many many years before the time of troubles.
As for the cause of the time of troubles it was Bane and Myrkull who stole the tablets of fate and enrage the Father of the Gods.
You can read the story in the Avatar trilogy novels of the forgotten realms line!

Here is a link to the books:

Click Me!

You can also read a little bit about it in the Baldurs Gate/2 Instruction Booklet. The section "The Roll of the years"