Because of the Power of WIshlisting...
Seriously, though, there is only so long they can keep having his Throne be failing without coming out with a fixit (though that could be several editions yet

). I doubt he'll wake, as it would have to be handled veeery carefully fluffwise to have it work without sounding like certain writer's wet dreams..
'The emperor wakes up, see, and he, um , well, declares a new Crusade, and all the xenos decide on the spot that he's their boss, and worship him as a god. Chaos gets it's butt kicked because the Emperor is The Man, and repaints his armour blue, you know, just because it's better that way..'
Having him wake up, be horrified at the state of the
IOM and start cleaning house internally, while the
SM and Guard continue their bloody, heroic slow withdrawal could actually be quite interesting...