How can I make on topic
Almost any question that begins with "why" is asking people to explain the motivations of other people. I.e. it is soliciting opinion and speculation, and therefore "off-topic".
Here, and in general on all SE sites, questions should be worded as if they were being asked of a librarian. It should be the kind of question that could be looked up in an appropriate reference book, assuming such a book existed.
E.g. "What, if anything, did Agent Smith give as an explanation for why …?" would be "on topic".
"Why" questions can be tricky. As Ray notes it's easy to ask some such questions that don't admit an answer based on verifiable facts. Caricature of prototypical mega-bad "why" question, that sometimes does show up here:
why did I sneeze this morning?
Issues with that one [besides not being about politics], just to show what the problem is in general with such questions on any SE site:
You probably can't even tell if the premise (that I sneezed this morning) is true. Even if you can establish/assume the premise is true, you'd have to:
Either speculate some deep reasons [e.g. that I have an allergy] or write something trivial/superficial as an answer (user Clint Eastwood gave some such [amusing] answers in the past to such questions), e.g. "because you had a semi-autonomous, convulsive expulsion of air from the lungs through the nose". (That actually explains any sneeze, as it's the [Wikipedia] definition thereof...)
So, as Ray noted, if you find yourself considering a generic/broad "why" question, at least give some hints in your question how you think it could be answered objectively, instead of [with] mere speculation and/or opinion.
Another issue with your question is that it's rather loaded. I mean it's not very far from:
why are your such an incompetent?
Your question title and your actual question diverge fairly substantially in scope. The title is
Why didn't the FBI monitor bankrupt (former) employees more watchfully?
Is this supposed to be a generic question to answer? Or do you expect an answer just about that particular investigation?
Wasn't it obvious that (former) bankrupt employees can be seduced into treason and espionage? Why didn't the FBI or U.S. government audit them? I'm surprised they didn't catch Ronald Pelton earlier.
You are basically implying that the FBI was terrible at catching any such spies/traitors because you go from a single case to asking a generic/broad question, implying they were generally incompetent in dealing with any and all such cases of "bankrupt (former) employees". Is an answer also supposed to survey similar cases and rebut the wider implication/premise of your post/claim?