Warning, Fanon ahoy!
(I spent a fair bit of time thinking about this during ME2.)
Miranda was assigned to keep Shepard alive before the Collector attack. But how do you prepare for the unexpected? How do you make sure that someone doesn't die, when you don't know what might be trying to kill them?
You don't. You simply make sure that they can be brought back.
Death is irreversible because of death of the brain - the loss of all the information stored within the game. It's like burning down a library - you can't rebuild the library if you don't know what was written there.
Brain death is irreversible - so Miranda decided to prevent brain death from happening to Shepard - no matter how the Commander might die.
In the month following the end of ME1, Cerberus arranged for Shepard's helmet to secretly be replaced with a new prototype. If Shepard's vital signs fell below a certain limit, the helmet would engage a stasis field around the Commander's brain, preventing the very processes that would usually lead to brain death.*
Unfortunately, when the attack happened, the system took slightly too long to kick in. Shepard suffered some brain damage and loss of information - this is why the Commander can come back from Lazarus as a different person.
Between the brain damage and the level of physical damage to the body, Shepard's reconstruction took much longer than originally anticipated. As a result, and because of the attack on the Lazarus Station, TIM decided that the cost had been too high to try again should Shepard die a second time (hence why an in-game death is still a "Game Over", not a "Let's rebuild the Commander again!").
EDIT: Regarding the planetary landing... it didn't happen.
Honestly, I haven't seen a single canon source to say that the Commander actually did hit the planet. Yes, you see atmospheric flare around the body at the end of ME2's intro, but it's just as easy to bounce off an atmosphere as it is to fall through. (Also, Shepard really shouldn't be anywhere near the atmosphere at that point, judging by the perspective of the camera and the relative sizes of Shepard and the planet, but now I'm being picky.)
Shepard floated in space until being discovered by the Blue Suns. All those broken bones you see during the reconstruction? Some were from the Normandy explosion and... I don't know... some come from the Blue Suns' ship hitting Shepard? (How did you think they found the Commander?)
*Unfortunately, my explanation stumbles slightly at the "Shepard is clinically brain dead" line on Kronos Station in ME3. I think I have to assume that particular Cereberus scientist wasn't in possession of all the facts...
Modifié par JasonShepard, 05 février 2014 - 01:59 .