In the course of his reading, George Orwell probably encountered certain storytelling conventions over and over again, and these are the devices he would have most likely used in his work.
Conclusion (that is why)
You can't be sure that Orwell read the futuristic books that resemble his own 1984.
Evidence
It's possible that the resemblance between his book and those futuristic ones is just that all the others were drawing on the same body of literary conventions that they had been exposed to over the course of their own reading.
Evaluate
Since we're told that the passage illustrates a principle, it means the principle is implicit, and it's our job to name what's going on, in a general way, like when we're doing a Parallel reasoning question.
The author is basically presenting a Curious Fact: why is Orwell's '1984' so similar to these other futuristic books?
She is saying we shouldn't necessarily jump to this Explanation: he read those books (and incorporated similar stuff into his own book)
Why? Because an Alternate Explanation is possible: he and those other futuristic authors were all exposed to the same body of literary conventions (and since they all incorporated that stuff into their books, their books feel similar)
Goal
Let's look for something like, .