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.