That doesn’t feel like the true reason that happened. Feels more like performance issues.
It kinda works both ways—in order to make 4 player coop functional, you’d require so many tyranids on screen that it would be unplayable, both hardware and for the players brain.
I know fanboys would scream if the marines were squishier, but what’s always confused me is that despite the legendary invincibility of space marines, in tabletop they still feel moderately squishy. Sure not like tyranids and orcs and such, but not like super human either. I suppose terminator squads with custom gear might feel closer to that mark.
All relative I guess.
It’s important to remember that the lore books are intended to be tinged as propaganda by the narrator. Of course the imperium will say it’s angels of death are semi-gods of the battlefield, but in reality armor only stops so much and a 7ft tall slab muscle walking tank is impressive, but a carnifex is still gonna carry its weight on the field as well and fuck up some astartes