(4) Base Camp
Asimov once said, "Everything I write is intended to enlighten, even my science fiction" (source). And that's why we're giving this a 4 on the Tough-o-Meter: Asimov probably knew a lot of big words, but he doesn't use very many of them because he wants people to understand what he's writing about. He also knew his audience was probably made up of young people, possibly even younger than himself. (When "Robbie" was published, Asimov was only 20 years old.) Add to this the fact that Asimov was an immigrant—he came from Russia when he was three, and he could speak English and Yiddish, and his parents spoke Russian to hide secrets from him—so he was definitely aware of how language can be a barrier to communication. This might help explain why Asimov tries so hard to be clear.
So, why not a lower Tough-o-Meter score? For two reasons: one, Asimov may try to be clear, but the 1940s was a long time ago, and things change. So, Asimov might say something that would be clear to his audience then, but might not be clear to readers today. And two, even though Asimov writes clearly, the ideas that he talks about are pretty big and sometimes hard to grasp.