If it had been Bungie's decision the first Halo game would have been called Halo simply, but Halo designer Jamie Griesemer told EDGE (via CVG) Microsoft insisted on adding "Combat Evolved" as a subtitle.
"Oh, man... the subtitle," Halo designer Jaime Griesemer told Edge Magazine. "At the time, Microsoft marketing thought Halo was not a good name for a video game brand. It wasn't descriptive like all the military games we were competing with."
"The compromise was they could add a subtitle. Everyone at Bungie hated it. But it turned out to be a very sticky label and has now entered the gaming lexicon to the point where articles that have nothing to do with Halo get titles like 'Skateboarding Evolved'. So I guess in hindsight it was a good compromise."