I bought Halo for the PC a few years ago.  I mean, it was a good 5-6 years after all the hype, so I was hoping I could enjoy it...

First of all, vehicle sequences are usually a lot of fun... but Halo ruined the controls by having you "point" the vehicles rather than directly steering them.  I got used to it... but driving the Warthog was never as fun as it should have been.

The campaign was extremely repetetive.  It honestly felt like it would have been a good 5-6 hour game, which was ruined by some designers who just copy/pasted levels.  I recall about 2/3 of the way through, I would destroy a room full of enemies, go across a bridge, and be confronted with an identical room on the other side... and when I went through that room, an identical bridge... I almost quit playing after playing the same bridge 3 or 4 times because I didn't feel like I was accomplishing anything.

Overall, the game was okay... but it doesn't compete with Half Life, Half Life 2, System Shock 2, Deus Ex, or many of the other fantastic FPS's out for PC at the time.  If the game was just released on PC, it would gotten mid-high 80's for score, and probably would have been forgotten shortly after. 

(Hopefully you see that I'm not just trolling, I actually took the time to answer your question...)

I would suppose its because Halo perfected multiplayer, and 5 years after Halo 2 still no one else has been able to even match that in longevity. hell, what other FPS can you turn into a racing game where you only have the guns of the back of the cars to stop the other team from getting the most laps while holding the blue flag on a custom made map with over 16 people? And if you manage to get them out of the car you have to duke it out samurai style with two/three clashing swords? And there is only one gun in the map, the almighty laser, with which  takes a fairly accurate shot to control. meanwhile while racing your own team, the other team sets up traps and ambushes along the way that the the other team must constantly avoid with the flag.