Heres the explaination:
In one of the more famous of Hetfield and Ulrich's controversies with bassist Jason Newsted, the album almost completely lacks the sound of the bass guitar. The standard explanation for this combines Newsted's absence from the mixing sessions (where he might have asserted his opinion) and the lingering issue of his "newness" within the band following the tragic death of Cliff Burton in September 1986.
As said by the band in their magazine SO WHAT!, they wish that they could re-mix the entire album because the drums and guitar overpower the bass completely. Some believe this is because Jason Newsted was new to the band and they wanted to frustrate him, though many, including Newsted himself, have stated this is due to the bass lines copying the rhythm guitar parts too closely.
Jason Newsted filled in the role of bass player, although this cannot be told from just listening to the album, as the other three band members admitted to turning the bass sounds down to an almost indetectable level. (It is often speculated that this was done intentionally by the original members of the band as a reaction to the death of Cliff Burton, and also as a way to haze Jason Newsted..
Supposedly, some guy did an electronic analysis of the album and found that the frequencies that are supposed to be there for bass guitar are missing but I cant find any info on it.....