I really doubt that, at least for me it isn't. Blackwater Park it's not my (most/least) favorite album, maybe closer to the top (but near the middle). But, if you add the bonus tracks, you got 10 songs (let's say 10 Opeth songs, which its a good thing), more than the rest of the albums (from 5 to 9 songs). Plus: I think Blackwater Park it's the most popular Opeth album, and i think that many people (now fans, or maybe not) in that "era" knew Opeth thanks to that album, so it's like their "first experience" (which "marks deeply", in all order of things, to people). And the final and most important thing: You (Ozzloaf, and im not blaming you), did this polls, and i guess Blackwater Park is your favorite album, and that's it's clearly inferable from your previous statements, so let's say that it's more "easy" for you to eliminate song from the others albums...
Well that's my humble opinion, and i think that's a good "rational" explanation to this "phenomenon"...
I voted TLA