What makes it so insanely difficult to get HD's? How are tasks typically marked compared to the hsc?
You usually get what you get
raw. Scaling up marks is only a thing for a very few number of faculties, and usually its cause they were hard to begin with. There's also a number of subjects that get scaled down.
Jake over-glorifies me, but HDs are indeed that rare. For starters, you're doing 2 HSCs per year literally. Your study time is also cut down, and cause you have more commitments now cramming is generally necessary (as opposed to proper studying). And generally speaking you don't really get to cover every cornerstone of the course that well.
Getting HDs is also harder in some faculties than in others, again simply because of difficulty. Note that whilst scaling sometimes saves you with difficulty, this is not always the case.
Oh, and also extending on the 2 HSCs per year analogy, it also means that you get smacked with an endless stream of assignments. First three weeks are okay, and then you'd be lucky to have a week without assignments/exams. If you do, you probably have 2 or 3 tasks in another week. Basically, you're bombarded.