I think it has to do with the fact that people tend to apply, in the first place, a kind of threshold of relevance to serial, long-form storytelling which helps distinguish need-to-know information from extensions or elaborations upon already established material which can heighten one's understanding but doesn't form the crux of any of the story's key movements. KH fans have hashed it out pretty well over the past decade which games constitute a relevant entry without ever coming to a universal resolution on the matter, but Launchpad speaks to a second qualifier which comes under consideration: how well does a game present its material as both relevant and, perhaps more importantly, compelling?
The natural inclination we've all pretty much inherited from our marketing/advertising based society is that if something doesn't look or feel good, it isn't "essential" and we're trained to divert our attention elsewhere. I think there's a logical (if flawed) train of thought derived from this which has impacted the KH series, which is that the higher the apparent or perceived quality of a product, the more important and noteworthy it is. So when a KH title takes a step backwards from a technological standpoint, or is reportedly of diminished narrative/gameplay value, it fails to reach the threshold of perceived relevance: it can't be as important as KH2, because it's on the DS and it looks kind of janky and lacks the cinematic standard set by previous titles (CoM notwithstanding...and even that eventually got a full PS2 remaster to hammer home its status as a bit of mandatory reading). The logical conclusion is that it's some sort of "spin off" or "side story" which doesn't carry the same weight as a full-fledged, capital KH installment. Of course this argument is basically fallacious, but as Launchpad points out, it's something SE should have been able to anticipate from a mile away and which needed to be compensated for by really pushing these titles and clarifying what they are: something which may or may not have been feasible given that they were generally smaller projects with no doubt more restrictive development and marketing budgets behind them, but which nevertheless was the only way to have a chance to overcome the built-in bias of advertisement culture. One could also suppose that if the games were exceedingly good, word of mouth might have boosted the perception of their worth and value, but that didn't happen because they were and are commonly regarded as just passable (and some would argue not even that).
The problem with the HD Remasters is that none of them are standalone: the marketing apparatus surrounding them was never very strong to begin with, but it had the added challenge of clarifying what each package contained and why these Remasters were essential to begin with...which goes right back to the same problem of explaining that these "spin off" titles are not really spin offs at all, even though all of the cultural coding surrounding them indicates that they are to the common consumer. Think about how people typically regard Straight to DVD Disney film sequels: these are more often than not fully canon and could technically be described as essential to experiencing the fullest version of each of their respective stories, but are not seen by anywhere near the numbers who watch the original, because the perception of them is that they are lower quality, ergo they are not as important or worthwhile of a viewing experience, so that even if a full theatrical sequel to The Lion King were to be released we would still observe audience members who not only had never seen but had likely never heard of the in-between titles which precede it regardless of their relevance to its plot. That's the power of perception which SE either failed to account for or didn't care to do so, and the Remasters are basically a Band-Aid prescription which technically serve their purpose but constitute a too-little-too-late solution to a problem which should have been addressed much, much earlier. When it's left up to fans and fan sites like KHI to direct other inquisitive potential-fans how to go about getting into the KH series, there's been a failure somewhere on the corporate trajectory from production to consumer.
How often do you ever hear people ask questions like, "How do I get into Harry Potter? Which book do I start with? Which movie?" Or Star Wars: people might, out of curiosity, talk about what it would be like to watch the films in their linear order, but implicit in that is how everyone pretty much knows that Episde IV is the "first" episode to watch, and how to progress from there. There's a clear communication of structure within these franchises which is almost unmistakable: the first because it basically follows convention and the second because, even though it just dodges convention, it maintains a clear progression within the tradition of trilogy format storytelling. KH, comparatively, decided to build its own little universe where none of the rules matter, and it could be argued from a variety of perspectives that it's a stronger or a weaker series for it inherently, but as a matter of mass consumption it has played right into the kinds of value conclusions people are inclined to draw about the products with which they interact.
Personally I really enjoy the "spin off" trilogy of Days/BBS/Coded, I think that experimental era was so essential to KH finding its footing as a series (for better or for worse) and I don't think we'd be seeing anything like the KH3 we're seeing now if it weren't for that period of franchise building (again...for better or for worse). For those of us who were able to track the progression of the series, I think it was an unforgettable and exciting time, to see the games get stretched in all of these various ways and try to establish all the ways they link together as well as what set them apart from each other. But from a marketing perspective, I feel this is 100% SE dropping the ball, and I don't blame fans who view certain games as "spin offs" anymore then I would try to convince them that Little Mermaid 2 is absolutely essential viewing. If SE/Disney wanted these games to stand with the numbered titles in the minds of the fans and carry the narrative weight of a mainline console game, then they needed to plan for that same level of exposure and follow through on it. FWIW I expect this all might have worked out fine in Japan where portable gaming was (is?) huge, so it's just the rest of the world's gaming industries which are probably perpetuating this misconception.
