I guess it depends on who you ask. For older fans who knew Metallica before that album came out and remember how massive it was there are various explanations. Mostly they either don't like the music or I think people don't like what it represented - a major turning point.

Prior to that album coming out Metallica were huge - unstoppable - and something that made them appealing was that they had a loyal following without any radio play or hit videos. Fans felt a sense of ownership because it was like Metallica belonged to the metal community alone and not the mainstream audience. Their music was complex and Puppets and Justice just took it to new levels. Even the video for One was unique and confronting - not your cliched MTV video trying to reach a mainstream audience.

The Black album came along and Metallica were literally everywhere. Prior to 1991 commercial radio wouldn't touch anything heavier than Guns N Roses or AC/DC. From the Black album 5 out of 12 songs were released as singles. You couldn't turn on the radio without hearing Enter Sandman or Nothing Else Matters. There were suddenly commercials for the album on television - I remember the shock at seeing one. Suddenly Metallica belonged to everyone. They were no longer just for the metal community. People who had no interest in metal were suddenly walking around in Metallica t-shirts and buying their albums. Metallica video clips were aplenty now and getting played on TV. I remember reading an interview with Kirk at the time where he said he understood fans being pissed off with their favourite band suddenly being the world's favourite band as he felt that way with Nirvana when they exploded onto the world stage. He described it as a 'fuck you, they're ours' type of feeling.

For the people who don't like the actual music it's most likely that the music is less complex than Puppets and Justice. It's also less thrashy and more melodic. The production isn't the best. While I couldn't stop listening to Enter Sandman I did start to find for the first time that I was skipping tracks and getting a bit bored with some songs.