So why have you never heard of it?
Despite the self-evident righteousness of the Pikes Peak event, hill climbs are more of a European style of racing that never fully caught on in the US. Plus, the Pikes Peak hill climb has always been overtly dangerous, which makes larger corporations a little skittish when it comes to sponsorship and participation. As a result, there's no one series of which the PPIHC is a part, as is the case with the 24 Hours of Daytona, the Indy 500, etc., which further reduces participation of the headline-grabbing drivers.
As for fans of the race? Only so many people (like yours truly) have the guts to stand along the twisting road as it goes up the mountain. And so it is that you've probably never seen or heard of the Pikes Peak hill climb, despite a century of history.