It's not a fact of "living in the past", it's a fact that they changed too dramatically, and it all happened as they started to become very well noticed in the media. It's as if they changed their style to suit the mainstream, instead of sticking with their original style to keep their true fans. This is seen as if the band is only doing this for the money. That is the proper definition of "selling out".
It's not that they've changed as such, either. Bands that change to become mainstream, or to adapt to a popular style of music, so they can fit in the mainstream music society and make more money. It's not just Metallica that are seen like this. Metal bands like Soilwork and In Flames have all changed their Melodic Death/Gothenburg style, which is only really known well amongst proper Metal fans, to Metalcore, which is becoming increasingly popular and gaining more air-time on shows such as MTV.
Other Metal bands like Dimmu Borgir and Cradle of Filth have changed from playing the proper style of Black Metal, into a more popular style in order to bring in money.
Metallica changed to Modern Rock, because in the early 90's, Modern Rock, Alternative Rock and genres similar to these were popular, and gained as much air-time as Pop Music did.
That is why people call Metallica sell-outs.
If you like their new style of music, that is completely fine. I am not judging you for liking it. It's entirely your choice, and I respect that. I'm simply explaining why Metallica have been given that label.
