How cognitive science explain the stylistic convention in text writing?
I don't think there is a stylistic convention in text writing that prevent the expression of an opinion. There are countless texts expressing a subjective view or controversial points. The example you picked up is somehow very specific to academia.stackexchange.com. 

The website explicitly tries to prevent the expression of an opinion - see https://academia.stackexchange.com/help/dont-ask about the possible reasons for a question to be flagged (and removed) as primarily opinion-based - e.g. a rant in disguise. This rule echoes the way science is made of - facts and interpretation -, especially in its formal way of communication. Researchers sometimes express an opinion in an article but it is usually advertised as such - see the specific section called "opinions" in the journal Trends in Cognitive Science for example. 

In the case of the removed sentence, I guess that the reviewer relied on "I would prefer" and "my opinion" to build his/her opinion.