Rumor has it that Steve Ballmer wanted people to refer to the new Xbox as "The One" because it was supposed to tie all these capabilities together and be the centerpiece of the living room. Additionally, Microsoft was going through a branding phase of "One" at the time.

As a personal pet peeve of mine, whenever a corporation chooses "One" for its marketing or branding it indicates an uncreative and unoffensive marketing team that can't think of anything more creative to convey a sense of product cohesion.