Why isn't this more well known?

Because people only tend to watch the newest stuff, or to only rewatch things that were new within their lifetime that they can remember watching on original release.

Is this a thread a joke? This is one of the most famous sci-fi movies of all time, next to Planet of the Apes, A Clockwork Orange and 2001. I'm 46, and the last line was referenced and quoted to hell and back when I was growing up, so of course people under the age of 40 had heard of it and the famous line.

Because it's the future. Now.

I think it fell out of favor over time because some of its design elements, especially the luxury apartments, became seen as dated. A fair number of 1970s films depicting the near future really dived into the modernist design styles of the late 1960s and early 1970s. This probably worked at the time to sell audiences on the setting representing a future evolved from our present, but over the years these designs have become dated themselves, which renders the vision of the future as dated itself and it kind of damages the film's verisimilitude.

I think it was also something of the product of its time when there was a lot more public acceptance/credibility about things like overpopulation, resource scarcity and environmental poisoning. Ironically, that same public concern led to environmental changes that have resulted in significant declines in pollution and in combination with improvements in technology have led to a general belief that global population growth is mostly sustainable and not the crisis it once was.

Most people are against remakes, but I think this one could probably be remade in ways that would make it more believable. It might help to have more world-building done, too, as I think the original was awfully vague in terms of the nature of political leadership.