>>535830383
The most common armour you used to see Marines in, Mark VII Aquilla does have exposed cable weakspots on the stomach but it is in fact intended to be a two-piece cuirass with a plate protecting the lower torso. The weakspots are in-universe ones because Aquilla was thrown together post-Horus Heresy when everything was fucked and they just wanted to get the production going as soon as possible. Earlier armors as far back as Mark III are like this, where Mark III upper plate extends downwards to protect the stomach, restricting mobility, main reason why it was changed later on.
Of course this is Watsonian, but Doylist they also figured at some point it was retarded and explained it away. It has been inconsistently portrayed in the past, mostly because video-games such as Dawn of War and Space Marine decided to cover the stomach with mentioned soft protection because it was just so much simpler to animate.
>>535830646
Nah, they got rid of the entire lower cuirass and it's just a shitty little plate in the middle. The "armored diaper" is gone too, so their lack of protection extends as far as their inner thighs. Phobos is even worse, because it has no plate at all. Primaris are a terrible fucking design and Age of Sigmar ruined everything for everyone, forever.