Because it presents a lot of things that are fundamentals to the genre, better than most if not all racing games before it. Things like best in class sense of speed, diversity and quality of track design, best in class graphics, best in class weather system, best in class audio reproduction, lightning fast UI and loading times etc. I mean, many of these are things that are paramount to racing games, and it does pretty much all of them better than any other racer out there.
On top of that, I think there's a large number of people who like me, appreciate the fact that it's a pure skill based, no frills, no nonsense racer. No narrative, fanfare, tuning options, racing line aids, rewind etc. It's literally all about genuinely learning the tracks and becoming a better racer, with no tertiary methods of support to make your life easier, besides simply getting better at the game. That in itself, coupled with the fantastic driving physics, and the aggressive and challenging AI, really makes for a racer that is in many ways more rewarding than most, provided you're willing to invest the time of course.
One review said Driveclub resembled a turn "back to the dark old days of racing games", and in some respects that is true, but those were never the dark days, they were some of the best, and that's also why the game is so refreshing.
