Think about it. In the lore, as of the beginning of the 42nd Millennium, the Adeptus Astartes are getting new weapons, new armour, new gear, and new enhancements to help them combat against the Forces of Chaos after the creation of the Great Rift. Meanwhile, the Astra Militarum are stuck with the same tanks and lasguns. I'm not exactly a payer of the tabletop game (in fact, I've only played Dawn of War, Dawn of War 2, and Space Marines), but shouldn't the Imperial Guard get better lasguns, better tanks, and better wargear in general, too? Sure, it would still pale in comparison to the new equipment the Space Marines have, but it's better than nothing. And this is coming from someone who's slow to adjusting to change.