1) The broad question that might be being asked here is: Why do people like dark art?
2) The specific question added on top of the first would be: Why has Nine Inch Nails achieved so much more resonance with critics and the public than the vast majority of other artists seen to be in the same sphere as them (him really)?
I'll try to answer the latter:
1) Outstanding command of modern technology in music - There are very few frontmen in history with the off-microphone skill that Trent Reznor has.
2) Ability to dream up and implement grand statements - Several of Nine Inch Nails' albums are just breathtaking if you listen to them start to finish. Even if you don't enjoy them, the wallop is undeniable.
3) Ability to capture emotion well - Nine Inch Nails has always seemed more attuned and sensitive than the industrial scene it came out of.
4) Desire to adapt and the versatility to do so successfully - You can't listen to just one album and understand the longevity because the longevity has come from not doing the exact same thing over and over.
5) Sex appeal both in Reznor and in his music - If one were to think of industrial music as a hammer, Nine Inch Nails is more of a needle.