"Why is he climbing like that?" The question echoed through the minds of the rest of the fallen geniuses as they all found themselves catching up to Ryuk.
While Ryuk was climbing the rope, they were all running on it, easily surpassing him, and they wondered why Ryuk wasn't running on the rope instead of climbing it.
Quickly, the blue energy manifested itself, slowing them all down as their speed drastically reduced. But Ryuk kept the same pace, climbing with all of his speed and power.
Sweat trailed down his face from the burning lava heat below him, but his eyes remained deeply focused.
Ryuk's reason for choosing not to cover his skin in Apex Energy was simple: He wanted to feel the heat of the lava. It was supposed to be one of the difficulties of the Infernal Rope Walk—the terrible heatwaves from the lava.
While the others were shielding themselves from it with their Apex Energy, Ryuk wanted to use his physical body to bear the full brunt of it.