Because they stuck around for thirty years and have made good music. They've played everything from thrash to lighter music. The heart of Metallica's success is the constant touring they've done throughout their career. The number of albums they produced is less compared to the span of time they've been around.
Metallica may have gone off to Never-Neverland if it weren't for the Rolling Stones. Lars Ulrich revealed that after a huge rift in 2004, the band nearly broke up for good. Metallica never sold out to the industry, they became a competitor in the industry.
You'd expect that the richest man in a band is the frontman, but going by Celebrity Net Worth, singer-guitarist James Hetfield's estimated net worth is "only" somewhere around $300 million. Its eponymous fifth album, Metallica (1991), the band's first to root predominantly in heavy metal, appealed to a more mainstream audience, achieving substantial commercial success and selling over 16 million copies in the United States to date, making it the best-selling album of the SoundScan era.
Like a lot of metal fans, I have complicated feelings about Metallica. But unquestionably they are one of the greatest metal bands. Their impact on the genre puts them in the top 5 as does their massive fan base and fame.
See the real life story of Metallica below!