Comment by danadam

To me, as non-native, it seems that it may be because the idea [1] of "cases" was already introduced. So, at the start of a conversation you would say:

- The rules of a/the are simple.

- Not really, if you think so, then you haven't considered _all cases_. There's this exception and there's that exception. [...]

but:

- The rules of a/the are simple.

- Not really. There's this exception and there's that exception. [...] If you think the rules are simple, then you haven't considered _all the cases_.

[1] but take what I write with a big grain of salt, I can't explain why I wrote "the idea" or "the start" and I'm not 100% sure if that's correct :)