At the same time I'd argue that a good student could find depth and sophisticated ideas to analyse in any text
That being said, I see where you're coming from; my school and quite a few other weaker English cohorts choose texts like OTW because they're very basic. This is further compounded the fact that they've realised poor English students can't be bothered reading a novel/are unable to analyse one effectively, which is why nearly 20% of the state writes on On The Waterfront.
Note that I said "easier" - I also think that a good student could write on any text and potentially still do well. It's just that I for one would struggle to find enough meaning in, for instance, "The Old Man Who Read Love Stories" to write a 10/10 on it; it's just a lame text with little depth. A 10/10 is conceivable, just harder. :p
Something else to consider perhaps - it might be the case that writing on some texts will make it easier to get TOP tier marks as well. For instance, few schools which teach texts such as "The Old Man Who Read Love Stories" get very high English scores consistently, and so examiners (who have no doubt noticed this trend in the past, since they're mentioned in the examiners' reports) might subconsciously be disinclined to give an essay on said text a 10/10 or even a 9/10 as a result; after all, it would have to prove itself as an "outlier", rather than as the norm. In contrast, Shakespeare (as an example) is often taught by the best schools, and so an examiner would expect to find 9/10s and 10/10s, and would thus more likely give them, perhaps.
I know it's lame to play the guessing game like this, but speaking as somebody who has seen firsthand how subjective marking can often be influenced by psychological factors (both from my own experience as an English marker and student, and even in things such as debating) , I think this is appropriate (and even reasonable) to point out.