When people think of the Halo series, they’re quickly reminded of the college dorm-room deathmatch. Halo is the quintessential multiplayer experience on consoles, but it wasn’t always like that. Before 2001, Halo meant nothing to people. It was just another FPS game that Microsoft was using to launch their first console, the Xbox.
To really get players talking (and ultimately spending their hard-earned money), Bungie had to create a compelling single-player campaign. If the core game was bad or run-of-the-mill, no one would care about multiplayer. And a launch game’s success is usually dependent on word of mouth. Look at Wii Sports. The more people that enjoy it, the more they talk, and the better it sells (which basically means more people to play multiplayer with).
Contrary to popular belief, Halo was originally all about the single-player, story-based campaign, which is the complete opposite of what it is now. → The happiest post on Earth.