It is the Hugos, which are voted for by fans who happen to buy a Worldcon ticket, as such, it's more about "what's popular" than what's the best written, most ambitious, etc. People vote not on literary merit, necessarily, but on what they enjoyed.
I haven't read Redshirts yet (it's on the list, especially now), but judging by other Scalzi work, it probably fits into the sweet spot of both fun and easy-to-read. I haven't read 2312, but I've always bounced hard off KSR, Alastair Reynolds can be hit and miss and I haven't yet tried Blue Remembered Earth... Had I voted, I probably would have voted Fractal Prince, which I really liked, but it is by no means an easy-to-read book. The series is a bit tough to get into, and it's a sequel, so you not only had to get through the first book, but like it enough to go through a second.
If I had to guess, Hydrogen Sonata's late release date probably harmed it, it came out in October of 2012, towards the tail end of the nomination period. I also haven't read it (maybe the biggest reason I shouldn't be voting, I'm usually a couple years behind the latest stuff and more often than not will wait for the paperback) but I planned to.