Simply because Opeth aren't very personal with their fans. Its why my favorite artist of all time is a female pop artist, who's taken the time to actually talk to me and other fans, and I like that. Opeth though, is my favorite metal band of all time, and is my 2nd favorite artist of all time, next to the pop singer, and Dark Tranquillity is 3rd, Novembre is 4th, Cradle is 5th.
But I really don't think there is a metal band that could beat Opeth, besides Novembre, and the thing about Novemrbe that holds them back is their new albulm had kinda sucky muddy production, and Novembre only has 3 good albulms to me, where Opeth has 7 amazing albulms.
P.S. Ghost Reveries was just as good as Blackwater Park. Watershed is like everyone says, but I have no clue why Ghost Reveries gets that tag. I feel its because many people heard 'The Grand Conjuration' first, and based the albulm off that, where me, the first song i heard of Ghost Reveries (also the first song I heard from Opeth) was Ghost Of Perdition, and if that song isn't and amazing work of art, nothing is. It sounds like anything off of Blackwater Park, but there's a new air of evil to it, and it sounds like a different color and stuff.