The scouring.

[Hyaenidae]

Quick note, Mk II plate was designed to survive surface and void, and was created specifically for these reasons; the Emperor was going to conquer the Galaxy, and Legionaries needed to fight in toxic hellscapes and void conflicts/boarding actions.. Mk III was created because of an additional need of forward facing warplate in the narrow hellholes of a boarding action / Zone Mortalis combat theater, Forlorn Hopes, or pure frontal assaults. It was never designed to replace Mk II plate, only supplement those Legionnaires who would benefit from such additional plating. The suit itself is clunky, loud, slow, and restrictive, and as such, is not suited to many different types of conflict where speed and/or stealth is needed.