: Spartans wear the armor that they think will be necessary for the job, unless
: they have special needs like Emile. Preferences come into play of course,
: like how some choose EVA instead of CQB because it offers better
: peripheral vision, at the cost of armor protection.

: Emile is one of the unfortunate casualties of this choice. A helmet designed
: for CQB may have saved him from injury entirely when he was deployed on
: [Redacted]. Unfortunately, the EVA visor shattered on impact, shredding
: the upper half of S-A239's face. Despite the UNSC's best attempts at
: reconstructive surgery, Emile was left blind without prosthetic implants.

: Beneath his EVA helmet, he wears a specially-built OCULUS rig which feeds him
: a composite image from four cameras, granting a 270 degree view. When
: another helmet was damaged in an Insurrectionist ambush, he carved a
: demon's skull into the visor, leaving the glass over the cameras
: untouched.

Of course they can change anytime, but in addition to making the Spartans individuals rather than look alike such as the early canon stated, I'm thinking Bungie did this so each Spartan character could be more easily identified by helmet style. Like the Spartans in the Marvel Fall of Reach comics or Halo Legends' The Package.