Originally posted by Classic NES
Because he uses Iai-style or Quick Draw sword style. Which is categorized by attacking with a blade from the sheathed position and then returning the blade to the sheath immediately after the attack. Zoro performed it so fast that you didn't see the attack, which is why it looks like he didn't do anything.
How is that a cool attack if you can't even see it?
That's like painting a canvas all white and saying it's awesome because everything is moving so fast in the scene that you can't see them.