Or more like because it's a hax than a physical force stuff. You see, most of protective enchantments are fidelius, disilusionment charm, intruder charm, anti apparition jinx, dark mark repelling spell(which bill modified and the death eaters has their dark marked people only barrier spell) that hides the location and it's visibility and automatic warning strangers, teleporation negation etc besides physical shields spells like protego. So when they get ride of all the other protection spells that conceal and alert them from attacks, they could just break through the physical shield that can be took down by firepower.
