They kept up-gunning them to keep up with power/feature creep elsewhere. Plus it felt like every army was almost tailor to mulch marine-equivalent units. Regular marines needed to be closer to Custards or Grey Kniggits rules-wise to fit their lore description. And I think going the Resartes route would cause less backlash than simply buffing regular marines to where they probably should've been. Marines should be like 3rd ed Necrons [or my memory of them at least]; relatively few in number, relatively short-ranged [but powerful] weaponry and a complete 'mare to kill. Remember when nothing in the Marine arsenal went past 48" in range? I remember.