There's a large segment of the playerbase that hasn't finished the fight yet because Halo 3 never made it to PC.
Keep in mind that the Halo community on PC isn't dead, either. SPV3 just dropped recently, Halo 5: Forge (a seriously watered down version of Halo 5 Multiplayer) is out, people still play Halo 2 and Halo CE on PC, and, in spite of Microsoft telling them not to, modders went and got Halo Online up and running for NA audiences (that is, until MS shut down Halo Online). PC audiences are thirsty for Halo, and MS better provide.
It seems rather bizarre to me that the Coalition managed to port Gears Remastered to PC, yet 343 is so hideously incompetent that that they didn't do the same for Halo 2 Anniversary, or even ensure that the game they ended up releasing was even working properly on release.