Ok, here goes my attempt at it then:
I think science fiction is the hardest genre to write, because of keeping the universe believable. The genre is defined by a new scientific breakthrough and showing how it changes life for the person or people that interact with it. Really, science fiction isn't supposed to show just humanity in space, it's supposed to be a genre about coming to terms with something never before experienced.
With that said, it is fiction, or the plot device isn't usually real, at least at the time of writing. Whether it's a submarine capable of going to the bottom of the ocean and revealing to humanity ancient beings that are beyond our experience, or meeting an innocent extraterrestrial being and being forced to defend them, you have to leave behind your grasp of the world and reach into the unknown. The difficulty in writing it comes from keeping the rest of the universe in tact and real to the reader. Humans are still humans, so you can't just focus on one character. Interactions with new things change everyone, for good or bad. The first humans at sea for more than a month likely went mad from the experience, and every new scientific achievement of comparable scope will cause such things. I'm going in circles here, but just imagine being on the first shuttle to mars, being with the same group of people for years, no where to separate yourself from them and come to terms with everything.
I guess what I'm saying is that science fiction requires an amazing understanding of humanity, and incredibly in depth character creation abilities. More so than any other writing, in my opinion.