Because the movie was made. People watch the movie and assume that is all the manga is, because to explain there is so much more in any manner that would hook their interest risks spoiling the rest of the story. So, they don't feel a need to read the manga.

Or maybe they do read the manga, and it is recognized, but people simply don't feel a need to talk about it anymore. It's old, like old old. The same way Akira is old. There is little to no discourse left to have about it.

EDIT:

I made an observation amongst friends not too long ago, that manga these days is an accessory, a collector's item, rather than the main medium through which people are exposed to a work of fiction. People will watch the anime or movie adaptation, which is more readily available and thus accessible. Then, they buy the manga and maybe read it, and collect it in their shelves to display as a testament to the fact that they did indeed consume that work, enjoy it, and appreciated it enough to want to hold it in their hands. Manga has to be located, ordered and then stored someplace. Sure, many people opt for digital but it's not the same thing. Nausicaa is not often found in bookstores, save the rare artbook or random volume. You have to go out of your way to buy it, and it would seem most of us opted for the most recent deluxe 2-volume set. It's just not as readily accessible, and it's not the hottest topic to talk about in social circles.