Why it is better to not use rhetorical questions in a research paper.

Because research papers are typically for the research and your finding on the subject.

Rhetorical questions add a level of cheesiness to the paper and unless you are a very good writer, and I mean college professor level, then you will most likely fail at any attempt. Also writing research papers your suppose to be telling the reader what they don't know instead of asking them what is already do know.