I think what it lacks isn't technical, it's hype. When it came out, no one was out in the streets dragging people to go see it, like they should have been. I thought for years that it was a silly action movie with good fx, until I saw it and realized it was about so much more. To be a classic something has to get ingrained in the public consciousness enough that people feel safe making references to it ("I can't do that, Hal"). District 9 just isn't there.

It could be, if it gets some sort of groundswell support by rabid fans (plenty of things flop and then later become classics, like the movie Clue). But it's not likely.