I think part of the problem with at least hard sci-fi for YA is that most YA readers haven't yet been introduced to a lot of the concepts hard scifi is based on. (I mean, most of us don't have entry-level physics courses until maybe junior year of high school, right?)

This isn't to say good hard sci-fi can't be written for YA, I just think perhaps it makes it more difficult to A) do well and B) gain enough of an active audience to be able to make it profitable