Because it's a storm shield sheathed in a power field and in the tabletop it gives a 3+ invulnerable save, which protects you against - you guessed it, meltaguns. From the codex: A storm shield is a large, solid shield that has an energy field generator built into it. Though the bulk of the shield offers physical protection, it is the energy field which is more impressive, as it is capable of deflecting almost any attack. Even blows that would normally cut through Terminator armour are turned aside with ease by the protective energies of the storm shield. A storm shield confers a 3+ invulnerable save. Oh wow, on a 6. That's 16.6% of the time. For a Titan class superheavy weapon. Amazing. For reference, a lasgun pistol will bypass a terminator armor's armor save on the same average. Marines don't die in TT by the droves, MEQ (marine equivalent) is the standard by which effectiveness of a weapon or unit is typically weighed due to their ubiquitous statline. Stormshields also usually cost more than MEQs, or about 3/4 the cost of a veteran, which is the only MEQ that can equip a storm shield in TT. They're usually only taken on close combat units since ranged units can just use cover to get 5+ or 4+ ruin cover saves. Bikes can just use jink saves in a pinch. The units that benefit from Storm Shields in terms of cost effect are independent characters such as Chapter Masters, Thunderwolf cavalry, Wulfen and Assault Terminators due to their close combat orientation and base cost per model. GW actually increased the cost of Stormshields on Terminators to the point where they're taken sparingly at best in 7th edition.
I thought the shields in EC were supposed to be like combat shields not storm shields, since storm shields are what the terminators carry. Combat shields are lighter but easier to carry, hence being used with power armor. IIRC combat shield gives a 5+ invuln while SS gives 3+
If anything I'd be happy that the storm shields protected against everything else in the game but hey, if we're going to go down the path of "MUH TABLETOP" then there are way more pressing issues in regards to other weapons, spells and equipment that needs to be addressed before fixing the still rather functional storm shields. But hey, what the hell do I know?
But on chaos you have other weapons with penetration to counter shields, like AC, PC, PG, PP, gravs, fist - eldar have none of it and the options we do have are either invalid(starswarm) or equally dangerous for user(tempest).
Shhhh! Everytime someone distributes stuff directly taken from GW material, the echoes awaken their army of malnourished Nazgul lawyers that voraciously sue the souls off everyone in sight!
