Thanks for trying to edit to get it reopened! Sorry no one explained in the first place; they probably thought it didn't really need much explanation. I did go ahead and edit again and put back some of the stuff you removed though. Context isn't what makes something like this opinion-based, and in fact the context helps explain what you actually mean by getting the most out of the peppers. (In particular, explaining that the heat is fine but you want more chili flavor is a lot less vague than "improve flavor".)
Anyway, as for how it's opinion-based, I wasn't one of the original close voters, but I can think of a couple possible issues. First off, totally not your fault, but some people might've mistakenly thought you were asking about the dish called chili (a meat stew). If you were asking about that, it would indeed be really subjective.
As for the actual question, I think the big issue is really that there are a lot of things people think improve flavor, and asking people to recommend them often does bring out a lot of opinions. Some people might like the mellower flavor extracted after long cooking, some people might like the brighter flavor from adding fresh peppers right at the end. Some people might think vinegar brings out the flavors they want, some might prefer salt. This leads to a mix of subjectivity and broadness, both of which can be reasons to close a question: there are a lot of answers, and some of them may be more about personal preference than anything else.
So, should it be reopened? I think my answer is a cautious yes. I'd like to see whether others agree first (not just use my binding mod vote to do it) but it seems like there are some reasonably objective things that we can suggest here, and hopefully it won't end up too broad.