Halo 4 is Microsoft Studios best-selling game of all time
Halo 4 is Microsoft Game Studios' top-selling title ever in the U.S. market, outselling its predecessor Halo 3. This is according to good ol' Microsoft themselves who noted that this is based on "Microsoft retail sales sell-through data for each respective fiscal year."
Presumably, this means that Halo 4 has also performed better than other Microsoft Studios launches, such as Forza Motorsport 4 and Fable 3. Microsoft will look to continue the success with Halo 5, which will be making its way to the Xbox One. It's biggest hindrance could be a lower Xbox One install base than what Halo 4 enjoyed with the Xbox 360. It's not yet known if Halo 5 will be headed to the Xbox 360 as well as Microsoft's next-gen console.
Given the success of the franchise, it's not surprise that Microsoft wants to expand "The Reclaimer Trilogy," originally planned as Halo 4-6, into "more of a saga."