It should be equally as difficult to achieve a certain band for English (Standard) compared to English (Advanced). DO NOT BE FOOLED by assuming that a low percentage of students achieving higher bands in English (Standard) is a result of it being difficult to obtain. The low percentage is a reflection of the candidature PERFORMANCE RELATIVE to the English (Advanced) cohort. It has nothing to do with difficulty of achieving a band. In fact, the performance band descriptors for English (Standard) and English (Advanced) are identical.
Keep in mind that both English (Standard) and English (Advanced) students attempt a common paper known as the Area Of Study Paper 1. This is where English (Standard) and English (Advanced) students are compared on a common scale (i.e. common content). Typically speaking, English (Advanced) students tend to outperform the English (Standard) students in Paper 1 simply because the strength of the English (Advanced) candidature is generally better than the English (Standard) candidature (in other words there are more "talented" students in English (Advanced) than in English (Standard) so the differences in ability show up in the results).
In the case of the Modules Paper 2, where the content differs, the distribution of marks in Paper 2 are adjusted accordingly to the distribution of marks in Paper 1 to eliminate differences in difficulty. This allows the marks to be comapred on a common scale.
So basically, the reason that there is a low percentage of students scoring higher bands in English (Standard) is simply because there are not as many talented students taking English (Standard) as English (Advanced). If for some reason, the English (Standard) students outperform the English (Advanced) cohort in Paper 1, then there would be a higher percentage of students in the upper bands. If you have seen the work of any English (Standard) students who have scored band 5 in the HSC, then you may have realised that their performance in the Area of Study is usually on par (or perhaps better) than the performance of English (Advanced) students who have scored band 5.