Well to be honest it's not actually that surprising, since doom metal has never been the most popular of metal genre's, as it's probably one of the most difficult to get in to.
It was pioneered by Sabbath back in the late 70's/early 80's. As you can guess, it's generally slow paced metal, which uses extremely heavy riffing and bass work, with the main aim being to crush and depress the listener.
There are quite a few sub-genre's, with the main ones being traditional doom, which uses clean vocals and is probably the most accessible, extreme doom, which is generally DM influenced and has growled vocals, stoner doom, which is psychedelic and very groove orientated and then you've got drone doom, which is generally ultra-slow, very repetative, but also crushingly intense.
Opeth to me have always sounded like a slightly more upbeat and progressive extreme doom band.
Some good doom banRAB:
Disembowelment (Death/doom)
Winter (Death/doom)
Confessor (Technical doom)
Sunn 0))) (Drone doom)
Trouble (Trad doom)
Witchfinder general (Trad doom)
Earth (Drone doom)
Electric wizard (Stoner doom)
Candlemass (Epic trad doom)