Well, you can only milk a storyline so much before it runs bone dry and people start looking for something new or different, hence why you see games like Halo Wars and Halo: ODST on the shelves these days....that's the developers trying new directions to keep the Halo franchise fresh and the fanbase interested.
No thats Microsoft being soft cocks and milking the product, Bungie was meant to finish the story arc at Halo 2, but was forced to release it early because MS wanted $$$. Bungie then had to stretch the last 3 missions they had to fit another game.
H:Wars wasnt even Bungie, that was Esemble Studios and they tanked after its release, probably due to releasing a RTS on a console without mouse support.
H:ODST was a expansion pack, but because MS still held the rights to the Halo franchise, specifically the Masterchief story arc, they made it out as a full game and shipped it out on full price, Bungie Studios even commented on it + it was developed in a year.
Halo: Reach SHOULD be the last Halo game from Bungie and they are going out with a bang. Since the game has been in development since H3 finished and after Bungie escaped their contract with MS.