I dont really consider Metal and Grunge to be all that comparable besides a group of loud white guys with guitars.
Like others have said Metallica had a huge, once religiously loyal cult following. The stars were aligned and all they had to do was not play so fast. I mean, Jesus, even Cobain was a massive fan.
Both bands, between their sound and content, served as the artistic elixer to that superficial Hair Metal fatigue plaguing the mainstream.
Whats more interesting to me is that Metallica was years of hard work and releasing revolutionary S-Tier albums finally paying off. Their album run in the eighties was the stuff of legends. On the other hand, Nirvana came seemingly out of nowhere and unintentionally brought a movement with them.
Both bands benefitting greatly from the stale, manufactured bullshyt that music fans had grown tired of it if they didnt hate it before. Plus they benefitted from a new generation of young people who couldnt be defined by the Reagan charged 80s glorification of greed, excess, and decandance. They needed a sound and a culture to represent them.
The stars were aligned.