Stack Exchange works because it's based on facts.
The moment you allow opinions, you open the floodgates.
Let's look at the history. It started with Stack Overflow. Stack Overflow was created in answer to Experts Exchange, but it also wanted to solve the problems with forums.
If you asked a programming question on a forum, there was a risk of the discussion turning into a Linux-vs-Windows debate. Or Emacs-vs-Vi / Java-vs-C# / etc. It would become a heated discussion, and you still wouldn't have an answer to your actual problem.
To prevent this on Stack Overflow, Stack Overflow went for a format where everything had to be verifiable and based on facts. It developed a special close reason for opinion-based questions because these resulted in, well, opinions. People would gain reputation points for having a popular opinion ("emacs is better") rather than having actual knowledge.
And that is another part of the answer - we have a privilege system that is based on points. The privileges must be given only to those who will yield them with knowledge - not to those who just have popular opinions.
There has been a lot of research based on people's opinion.
Great! If this is actual scientific research, it may confirm people's opinions, or contradict them. In both cases we have facts.
If it is just "research" where somebody searched the web for arguments in favor of their opinion... all other social media already have that. We don't need it here.