The SG05 uses an SFX power supply, which sits over the CPU. Changing it to an ATX power supply would cut down a lot on the already limited CPU cooler height.
So why not microATX with an SFX PSU? Well, up until pretty recently the highest available wattage SFX was only 450W. That's enough for a mini-ITX system, where you're limited to a single GPU, but microATX opens the door to multi-GPU configurations, where SFX won't necessarily cut it. It is more viable now that 600W SFX (and soon 700W SFX-L) are available, though it still won't be enough power for every scenario. It's almost exclusively Silverstone that sells higher wattage SFX PSUs as well, and consequently they're one of the few case manufacturers that does SFX-only cases. Most other case manufacturers are also in the PSU business, but they typically only sell ATX PSUs, so it doesn't make business sense for them to do SFX-only cases.