Just something I noticed, and it's been egging me for a while now.
Why do you think that is?
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Apart from the fact that Shepard's suit was ruptured and breached in space while falling from orbit, MJOLNIR is millions of times better in terms of armor quality and functionality. Apart from the fact that the MC also had a plate of Forerunner metal to protect his reentry, its pretty obvious as to why Shepard died.