nuance/emphasis Now I am no expert, and my view on this might be completely wrong, but here is my two cents.
When you say something along the lines of "〇〇じゃないですか" it can be used to mean something like "Isn't it **?". In this way it is not meant to ask for an answer to something you don't know, but rather a nuanced way of asking for a confirmation of what you are hoping the person you are asking to say. You are trying to ask a question with a desired answer already in mind instead of an open question.
Now you would normally say 良くなかったです instead of 良かったじゃないです, but in this scenario I think the じゃないです form would work just as well if not better. For trying to ask for confirmation in this way with some words you would conjugate it normally. For instance 行きませんか ("won't you go?") is the way you would ask for confirmation if someone is going somewhere in this nuanced form. However, with some words it kind of (at least to me) feels like you would do things differently.
Saying 良かったじゃないですか is sort of a way of emphasizing that you are asking the question in this nuanced form. In this way it's as if you are putting 良かった in quotation marks because you want the person you are asking this to, to say "はい、良かったです". You have a desired answer that you are trying to lead the other person to say.
If you say 良くなかったですか it doesn't really feel like you are trying to ask for confirmation, but are rather just straight up asking "Was it not good?". It doesn't convey any sort of position that you are trying to lead the answer towards.
Again this is just what I feel is the answer, and it may be completely wrong. Hopefully I explained my view point clearly.