I don't think anyone has the opinion that BvS is forgettable. Most people just think it's not very good. And others do think it's good of course.
I think BvS is continuously talked about for various reasons. The biggest reason I feel is a lot of hype that turned into disappointment (for some people, not all). Didn't we all know about BvS for almost three years until it's release? The movie was hyped and marketed by both people at WB/DC and fans as this soon-to-be modern pop culture phenomenon, and it didn't really deliver on that. It's one thing to say you liked the movie, but I think pretty much everyone can admit that BvS did not stick with people (including the general audience) because it was the great DC trinity on screen together for the first time. I don't think the first Avengers film is so revolutionary in it's story. It's pretty generic in plot and good enough in my opinion. But what stuck with people was the fact that these heroes they started to get to know for four years finally came together. Marvel Studios was able to pull that off with characters the general audience wasn't too knowledgeable about as they were about Superman, Batman and Wonder Woman. Can you imagine what it would have been like for the MCU is Avengers had failed? I should also add that comparing BvS to F4ntastic doesn't make such sense, because nobody wanted another Fantastic Four movie under FOX, and many saw it as a sly attempt to keep the film rights and as a cash grab.
Secondly (and I'm not saying this about anyone here) a lot of DC/DCEU/Zack Synder fanatics took it upon themselves to continuously take shots at the MCU and it's fans. This even extended to Zack Synder, Ben Affleck and Deborah Snyder as well (though I think hers was more like shade to Marvel Studios than an actual dis). The fanatics themselves kept saying on many boards that "the DCEU will show [Marvel fans] how it's done", "create a better cinematic universe", that the general audience is tired of "Marvel's kiddie movies", "people will take the DCEU seriously" and all of that. Zack Snyder himself called Ant-Man "flavor of the week" (something I do kinda agree with), Ben Affleck also said that BvS and other DCEU films "are more grandiose and can't be as funny or as glib as Marvel movies" and Deborah Snyder said that the DCEU films are more "director driven", if I am not mistaken, around the time Ant-Man (which infamously had directorial issues) had come out, or she just said it anyway and it was interpreted as a dig to Marvel Studios who has had issues with directors before. Whether or not one agrees with these statements is irrelevant because they said these things either way. Because of this, what happened to BvS (and the DCEU in general in 2016) was seen as karma for all those comments. It's a petty war between the fanboys of both sides of course, but Marvel 'won' at the end of it. Civil War does not get brought up repeatedly the way BvS did because it was another critical and commercial success for Marvel, while BvS, though an undeniable box office hit, seemingly performed below some expectations (the whole "it should have crossed a billion dollars" thing) and its performance was actually affected by the negative critical reception/word of mouth if I am not mistaken. Even Iron Man 2, which many of us in the fandom believe is bad, was not panned by critics. Not to talk of the persistence from the "BvS is too smart for you" crowd.
I personally don't like the film at all, but one can't deny that there's obvious reasons why BvS is still brought up unlike other bad movies. Whether it's just because it dared to be different I think isn't enough to say why it is. I think it's because it was hyped so much and disappointed a lot.