Well, the explanation is rather simple: we humans are creatures of habit. We prefer to stick to what we know rather than experiment with new ideas, unless something shatters the established order of things. This is somethings that pervades every single aspect of our existence. Case in point: have you noticed how the majority of fantasy settings have elves, dwarves, dragons and some dark lord up to no good? Ever noticed how nobody really seems to give two shits about that fact? When even our escapism from the daily grind is filled with routine, well, it certainly speaks volumes of our obsession with regularity and doing things by-the-book.
Of course, this entire phenomenon we represent was born of the order of things shattering. Something with a not-so-proud heritage of being utter tripe, suddenly made good, in a way that garnered a completely unforeseen audience.
The early fandom was composed of the more adventurous sort, folks who’ve truly grown tired of the established order and seek thrills ever more exotic. This, along with the relatively small group size, meant that information flowed much more efficiently and people were more receptive to alternative interpretations.
Unfortunately, that is a sate that cannot last. The most adventurous eventually tire and move onto new pastures, while others find their niche and settle into it, developing a routine and becoming attached to it. And in this sort of case, the more ordinary folk see all the fun we’re having and want a slice of the cake too. And so, what the pioneers (largely) agreed on has become codified into fanon, and most of the newcomers subscribe to these interpretations because they figure “when in rome, do as the romans do”. And since people like to fall into routine, they don’t really give alternative ideas that much credence, except perhaps for a one-off joke.
That’s the way the cookie crumbles, as unfortunate as it may be. Still, like others pointed out, new ideas do still flow, just not as well as before. People are more set in their ways and since there’s a lot more of ‘em, it’s harder to even reach a sizable portion of the fanbase with your ultra-cool new interpretation of Lyra’s funny sitting habits. Thus, most won’t even bother, since revisiting old ground is far more lucrative anyhow.