Since the release of Halo: Combat Evolved in 2001, Master Chief has become one of the most easily recognizable characters in all of gaming. In 2015's Halo 5: Guardians, the Spartan shared the spotlight with a brand new protagonist, Locke — and 343 Industries is aware that some fans were disappointed by this choice.
"People have become attached to him over the last fifteen years and they’ve started to sort of fill in the gaps that the character deliberately has for gameplay reasons with a genuine emotional attachment," said franchise manager Frank O'Connor in a recent interview. "We certainly underestimated that with Halo 5."
O'Connor goes on to state the studio's focus has shifted from introducing new characters to making the Halo universe "a little bit more realistic and compelling." There's a clear implication that Master Chief will be the primary protagonist of the series going forward, in response to demands from fans.
While the use of dual protagonists in Halo 5 proved to be divisive among fans, this kind of conflict is nothing new for the series. In 2004, opinion was split when players delved into Halo 2, finding that the campaign was split between Master Chief's missions and the exploits of the Arbiter.
The Halo franchise has always had grand ambitions in terms of world-building and lore, and its stoic, rigid protagonist sometimes seems to be at odds with those aspirations. However, the fanbase has spoken, and it seems that 343 Industries has no choice but to conform to their wishes.