I think the seeds of the New 52 Superman's demise were sown when Convergence came about, did a specific story featuring the Post-Crisis era Superman and gave that version a wife and son.
The response to the N52 Superman were always tepid at best and when the L&C Superman series came out (which put the Post-C Superman/Lois in the N52 reality), that had a lot of critical success and IIRC, was selling well too.
It was around that time they did the Unmasked storyline, where Superman's ID was made known, he lost his powers, ETC. That didn't gain as much traction and positive reviews as they'd like (the whole DC YOU line started with that story after Convergence and was quickly scrapped after it began), while L&C was still going strong.
Then came Last Days of Superman. Personally, I don't think it was meant to be a permanent death of the New 52 Superman. If you read both Superwoman and New Super-Man that launched after it, both of them are very much tied into the New 52 verse. I stopped reading Superwoman after #2, but I know after SUPERMAN REBORN, Phil Jiminez (writer/artist of the book) left because Reborn totally upended what he was trying to do.
Likewise, after SUPERMAN REBORN, aspects of New Super-Man don't make as much sense and there's been some continuity snafus already (as huge swaths of the New 52-verse got swept under the rug, but New Super-Man kept referencing them in issues afterwards).
I think what happened is that when REBIRTH happened and people loved having the 'old' Superman back, what was going to be a temporary situation until N52 Superman returned (as would his Lois), Editorial decided instead to just roll with what was selling and making people happy, giving readers what they obviously wanted.
OF COURSE this is conjecture, but IMO just as a student of comics history and how DC tends to do things (which is haphazardly, as seen in how Flashpoint was turned into a universal retcon story at the last minute when it initially wasn't), this is pretty much my theory until someone at DC does a tell-all interview.