For much of 40k, firepower has trumped durability due to the prevalence and access to high AP weaponry. Any of the AP systems and possible stat-lines could have fostered a more lore-friendly 'elite' Marine, where their durability is a significant differentiation factor to their play, but that will only happen if high-powered weapons are made much more cost/list-prohibitive to bring.
Bring the number of them down in a game so that on average a player is only actually able to fit a few in a list, with the majority of affordable weapons being small-arms and/or short range, and watch the battles become more about maneuver, cover and protection of those few high-powered weapons, rather than stand and shoot to the death actions where basic troops are useless, and the virtue of durability is an afterthought.
Take the high powered toys away, and 40k becomes a much better wargame. Some restraint is needed in the number of 'special' on the battlefield.
But as GW likes to say, they are a model company first, and such a design-ethos would go completely against selling more models and large LoW kits, not to mention might upset those who like the more fantastical larger-than-life model aspect of the game, or the idea of powerful weapons everywhere.