RT:
They Live - 83% critics / 95% audience
D R E D D - 78% critics / 93% audience
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"They Live" opened on November 4, 1988 and debuted at #1 at the North American box office grossing $4,827,000 during its opening weekend and had a total domestic gross of $13,008,928 (with estimated budget of $3,000,000).
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"DREDD" was risky enough with the budget and terribly marketed.
It did very-very good considering the rating, the 3D, the complete lack of screenings, and dumb ass comparison with "The Raid".
They are still underrated by the mainstream audience. The fact that you called them cult classics means they aren't mainstream films
Also, "They Live" has a 73% audience approval not 93%, and is by the RT users, only 36,814 people, that's not even 1% of the population watching films in theaters. Also it has a 83% critics approval, but it only has 46 reviews, far from the 200 + reviews modern films get. And if you look at the negative reviews, most of them are from 2013 and 2014.
*the audience approval seems to fluctuate. When I went it said 73%...then later 78%, so no idea what's going on*
They Live - Rotten Tomatoes
Dredd has a 78% critics approval which is not terrible, but it has a 72% audience approval, not 93%as you said.
Also, it cost $45 million and that doesn't count the marketing cost, and it made $41 million, which means it loss the studio money, which is why there is no sequel. It opened in sixth place with 6 million dollars. Regardless of the bad marketing, it had no completion, and Kick Ass was also R rated and it made more money.
But whatever the reason is that a film fails to reach a large audience, it doesn't change the outcome or the reality that they aren't mainstream and they are underrated by the majority of the population when compared to more popular mainstream films. Therefore, they are underrated.
Dredd - Rotten Tomatoes