He used a chunk of Forerunner metal as a surfboard on the way down. If you hadn't noticed, that stuff doesn't burn so easy. And he has actual power armour, a functioning, keyword here now, Sealed, mechanized suit of awesome that augments the wearer inside and has several powerful layers of defense. The outside energy shielding which activates upon contact with anything, in essence creating a protective sealed bubble around the user as long as the shield generator hold sup. The out layer of Titanium A battle plate which the UNSC favors, which has been shown to be more resistant to heat and plasma when compared to other metals. The black undersuit which is comprised of carbon fiber nanoweave type stuff, light but protective as well, and last but not least, the person underneath it having their own augmentations with partial bodies with un-breakable bones, incredible strength and stamina, and other things. Shepard didn't actually die from the fall, but rather, exposure to the vacuum of space. His/her suit was breached when those two [i]really handy[/i] oxygen tubes on the back of the neck were broken. As for armour, it isn't a fully mechanized suit, hell, you could hardly even say mechanized. Correct me if I'm wrong, but in canon, kinetic barriers only activate when high velocity physical objects come in contact with them, which means they'd be useless to shield against high temperatures on both ends of the spectrum, which this has shown before in gameplay. The armour itself is forged using mass effect fields, providing as much protection as possible from attacks related to weapons, making it super dense but light in the process. Overall it comes down to the fact that Chief's line of hardware was designed for more than just combat, eva activities, hostile environments, nearly bullet proof and plasma resistant, as well as having built in medical functions. And the fact that Chief used a piece of almost near indestructable alien architecture to ride down almost the entire way so he didn't take the brunt of the atmospheric traumas. Meanwhile Shepard was dead upon entry, and his/her armour isn't a full suite of things like Chief's is. These features have to be added on for special operations. His/her shields would have done nothing to protect them from the extreme heat, and their suit didn't have as many protective layers. They had no object in front of them to slow them down in the atmosphere or to absorb the heat, and they were only regular human. They probably spattered to pieces once they hit the ground, whatever was left of them.