I still wonder why it was so successful. Perhaps it was a combination of good timing and good marketing that brought all of the goodies that PC FPS gamers had enjoyed for so many years to a wide audience of adolescent teenagers.* I won't deny that Halo was a good FPS....but it's certainly not everything it's been hyped up to be, and there are definitely good FPSes elsewhere.
IMO, it was an evolution of hype that carried forward after it had started to get a little past the expiration date.
When the first teasers and footage for Halo came out, that was still back around the Quake 2 / early Quake 3 time frame. And Halo was still supposed to be a PC game. The graphics looked great compared to the state of the market, and Bungie had a good reputation. "Vehicles" were still some groundbreaking novelty, and the teasers were slickly produced - again, relatively speaking.
So it generated a lot of interest from the gaming crowd that carried over when the change to Xbox-only was made. Playing armchair psychologist...PC gamers decided it was already going to be the next cool FPS, so they had to own it. That mindset managed to survive until it was eventually released on a console, at a time when it wasn't quite as exemplary any more.