Thanks for your response.
I think my question can be rephrased in other word:
Why don’t the default setting use monospace since it generally looks better in a markdown editor?
To me, this default setting caused my markdown looking much uglier than monospace. By using monospace font, I usually spent >90% time in markdown editor mode when I edited things in github and my previous notes app.
My opinion is from personal 6+ years markdown-based note app experience. People might disagree though.