Garage Days Re-Revisited also had lots of bass on it, which makes the complete turnaround on And Justice for All even more bizarre. They're definitely a bit better about it now, but the simple fact is Metallica's guitar tones eat up a lot of the frequencies the bass uses so it'll never be as audible as it could be.
e: Also, let's not forget even Maiden's most recent release (the Live Chapter) is actually very light on the bass compared to practically everything else they've done. I'll be interested to see if this is a new trend or just a one-off resulting from Steve not being involved in the mix.