Metallica were on top of the metal world after just 3 or 4 albums. Opeth has been going for a lot longer than that.
I think its mainly because Opeth are a prog band, and Metallica aren't (despite their early prog influences). Opeth seem to have more of a massive cult following, more like Tool or something.
Comparing bands doesn't really lead to conclusions, especially with a band as unique as Opeth. Back in the day people called Metallica metal's Led Zep. In the future someone will probably be called the new Opeth.