Blame it on St Anger. Specifically Bob Rock.
He did great with "Metallica" aka "The Black Album"; had the right vibe to make it palatable for the mass, hence the popularity of the album. I personally like it. It was THE album which introduced me to Metal in general (was listening to Richard Marx before that :lol: )
Then "Load" and "Reload" came out which basically made them lose their footing. "Garage Days Revisited" was ok, and nearly everyone thought they are back on track when the "S&M" album came out (RIP Micheal Kamen).
Then Jason left, Bob's ego got big, Metallica listened too much to him, they tried to be experimental with their song writing and "St Anger" became a flop.
Thank goodness they are looking for a new producer for their next one. Not too sure who, but I am hoping it's Rick Rubin, heh. 8)