The reason for the low number of students achieving a band 6 in English (Standard) is because most students simply do not have the capabilities to do so. Remember that English (Standard) and English (Advanced) students are compared on a common scale in the Area of Study Paper 1 and there is no discrimination between them in this paper. Also, in the Modules Paper 2, the marks are adjusted to fit the distribution of marks in the Area of Study paper. In other words, if the top student in English (Standard) achieves a mark 90 in the Area of Study paper, then that is the highest possible mark that a student can achieve in the Modules paper (and overall). The same applies for English (Advanced). Therefore the aligned marks given, are on a relatively similar scale for both English (Standard) and English (Advanced). It is because of the common course content of the Area of Study, in which most English (Standard) students score lower marks.
It is still possible for someone to excel past 91 (which is the highest aligned mark on record so far) and even further to the high band 6 range. If someone is able to achieve a really high score [i.e. x > 91] in the course then that means that this person must have excelled in the Area of Study paper in relative comparision to the English (Advanced) students and has remained consistent in the Modules paper. It's just that no one has achieved it yet. If you can excel in the English Area of Study Paper, well past most of the English (Advanced) students in the HSC and continue to perform that well in the Modules Paper, then there's no reason why you shouldn't get a band 6.