Further, as several people have pointed out - they obviously want to show off the new body model / skin textures / GIANT ROID VEINS. Kind of hard to do that if every armor in the trailer & screenshots is heavy full-body stuff.
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Some of the more detailed pen-and-paper RPGs I've played around with, have quite a bit of variation in armors. Rolemaster, for instance, divides each level of armor (soft leather, rigid leather, chain, plate) up into smaller categories, based on the coverage. The lowest level of "Rigid Leather", for example, is the briastplate - it just covers the torso & shoulders; down to the hips, and a bit of upper arm. Now, normally, you'd wear that over a shirt of some kind..... but that's not all that different than a bare arm, to your average sword or mace.
"Realistically" not everyone had the resources for full body armor. Or they had a high-mobility fighting style that made them not want to encumber their arms & legs.
(I keep being tempted to do a Paul Hogan, and say "That's not skimpy armor, THIS is skimpy armor!
" and link to some of the thong bikini platemail mods.... but I doubt it would add anything to the discussion.
)