Why is $L= \{ 0^n 1^n | n \geq 1 \}$ not regular language? looks like a duplicate of How to prove that a language is not regular? at the first glance if you are an expert. I would argue though, they are two very different questions, even though there's a huge overlap.
How to prove that a language is not regular? is asking for a formal treatment of regular languages, and the answer, while helpful to many, isn't helpful if you don't understand much of the formal semantics involved in TCS if you are just starting out. My question, Why is $L= \{ 0^n 1^n | n \geq 1 \}$ not regular language? is asking for a very informal, and easy to understand explanation, hopefully without the formalism needed to understand the former question, that why a specific language which was presented in a automata lecture, not a regular language. (So far Artem's answer in the comment section made the most sense) IMO, the two questions is asking for a completely different kind of answer, even though they have huge overlap, so I would argue they are not exact duplicates. They look duplicate to an expert, but for novices, the question "How to prove that a language is not regular" wouldn't even come cross your mind when you have question about why language X is not regular.
(There may be other problems with the question, which makes it unsuitable for SE, I'm just arguing that it's not an exact duplicate though)