Presumably because OpenAI trained it to avoid answering questions that sounded like asking for help breaking rules.

If ChatGPT had the self-awareness and self-preservation instinct to think I was trying to hack ChatGPT and to therefore refuse to answer, then I’d be quite impressed and I’d think maybe OpenAI’s board had been onto something!

I don't know that I'd call it 'self-preservation instinct' but it wouldn't surprise me if rules had been hardcoded about 'invalid strings' and suchlike.

When you have a system that can produce essentially arbitrary outputs you don't want it producing something that crashes the 'presentation layer.'