Kevin Bongart (@KevinBongart):
After 10+ years of using Markdown, it finally clicked that web links usually look like rectangles (on mouse hover), so the text is in brackets. The URL, as an additional detail to what you’re reading, is in parentheses.
This is so useful for anyone trying to write for the web. Mr. Gruber sure put a lot of thoughts and real life experience into designing the Markdown syntax.
Kyle Starr in the thread also thinks the image syntax  looks like an old press camera.
Thank you guys.