I don't know quite if this is the case, but sometimes I get the feeling that maybe some fans are a bit like this:
They start off with a fairly new album, could be the latest or could be 3-4 albums back, but still a fairly modern album.
They start checking out the albums being close in the disco, I can imagine people starting with Heritage to work themselves back one album at the time, and people that starts with BWP probably checks the albums that has come out after.
Eventually there's the first 4 albums left, and I can imagine a lot of people checking out Still Life and MAYH because a lot of people here really praise them, so a bit of curiosity is not that surprising.
But then we get to Morningrise and Orchid, and as much as I like them, they have been in an uphill battle here on DTF since like forever, and I can imagine people maybe not check them out so much, just because they don't "need" it, or something.
They still have 7-8 other Opeth albums perhaps, and maybe they don't think it's worth shattering the image of Opeth as being very high quality and solid, by listening to an album that a lot of people diss.
Again, this is just hypothetical.
Another example we could do is Porcupine Tree, where most people start with probably In Absentia, Deadwing, LBS or maybe Stupid Dream.
When they get to know the discography post-Signify, I think there are quite a few who don't work their way back more, and it really shows in survivors and such.