Maybe because they'd need to explain who found him after the mass relays are destroyed. Or maybe because of the extensive injuries he might've got after walking into an explosion. Or maybe because they were forced to release the EC fast because of all the flaming. I don't know really.
The point I was making is that high EMS Destroy features more content that Control or Synthesis due to the breath scene. It has voice-over for slides, it has a memorial scene. It also has the breath scene which is something neither Synthesis nor Control have. And the argument for closure is subjective, because as you mentioned, breath scene is pretty clear about Shepard's survival. If anything I'd say that Bioware failed to create ambiguity about Shepard's fate.
Again the fact that people are complaining and not a small minority, that the breath scene doesn't give them closure is substantial evidence that the breath ending DIDN'T provide closure. Stating that it is subjective isn't relevant, many things in life are subjective. The EC did little to satisfy fans about the endings and it had zero effect on does Shepard live or not in the breath scene. These are facts given how often people complain about the endings and how often people complain about that breath scene. When the numbers are that high its pretty strong evidence that closure was not provided for too many people, subjectivity no longer matters.
While I want Shepard to be dead what I want more is closure with that scene. Ambiguity towards the protagonist is not a healthy way to end the story, when the story was always presented as a trilogy. You want people to end the trilogy feeling satisfied, far too many people end ME3 feeling completely unsatisfied yet they will turn around and say they "loved ME3 except for the ending." You don't want people saying WTF?!? you want them going WOW! They knew people were up in arms about this ending pre EC and they took the time to make the EC (Which they didn't have to), but it failed, as it left so many fan asking the same closure questions which they refuse to address or provide contradictory answers. I have heard of both pro life and pro death responses by devs to the breath scene.
Now I'll grant that this ambiguity could have been their intent but it was a bad choice if so, because of how upset it makes far too many fans. When you spend years on a series you want closure at the end, you want that satisfied feeling which is 100% independent of the issue of a happy or sad ending.
On a personal note I have no problems with head cannon but I don't want to head cannon the ending. I'd rather it be clear Sheppard lives or dies.
Also all the questions about how Shepard survived and situation of his rescue could have been swept aside with a simple statement "In the aftermath of reapers destruction rescue crews discovered Shepard severely injured but alive." That's all that is needed. Hell you don't even need to show Shepard, just an acknowledgement of one simple sentence solves all the questions of his rescue. And a better memorial scene would have been someone from the Normandy removing Shepard's name from the memorial. I'd have not liked that ending but I would have 100% satisfied feelings about said ending.
Maybe their intent was that Shepard should never survive the ending but they wanted one possible ending where people could have hope that he survived, so it wasn't all doom and gloom for those people who couldn't accept their hero died. Well that's an incredible stupid move as well. This isn't a Teen title this is a MATURE title which means they should be targeting their audience as adults not teens. Which means you should tell mature stories and if Shepard is dead full stop then make it clear he is dead full stop as we SHOULD be all adults here and that means we don't need adolescent hand holding for the sad bits of a story. Give it to us straight, and don't patronize us.
I in all honestly don't know why things happen this way. I think it shows the Casey has a LOT of maturing to do with storytelling, again maybe there were other factors maybe he or others were second guessing themselves and they got into a vicious circle and the result was the breath scene but once it was there they didn't want to nullify anyone's interpretation. I'd hazard a guess people would rather them say definitively what's what vs. any fear their head cannon turns out to be null and void(this is re the ending only, as head cannon is important but i think people want more of an "official" result for endings). The end result is that ME3 was almost universally panned for its endings and that has really angered a lot of fans, that ISN'T the way you want to end an almost universally loved trilogy.