The only reason is used here is because is the subject of a **relative clause**. Relative clauses don't have topics, so is used instead of .
In a main clause, the in would likely be exhaustive rather than neutral (because it would be weird to have a neutral attached to something already in the "universe of discourse"), but here in a relative clause that's not necessarily true.
The , on the other hand, is a plain thematic . It's a very common construction to have the topic follow a nominalization:
>
> It is good that you have quit smoking.
In other words, is used here simply because there's no reason to use .