Because the only thing Bruce has on him is strength (which is also can be debatable to a point), as mentioned above already. Arrow has him beat in tactical intelligence, speed, agility, durability, etc.
Off-topic, the real question is, who would win with prep, and if Arrow can create an EMP arrow.
Okay, first you say "Arrow wins" and now you start talking about who would win with prep time. If you think so highly of Arrow, it should be obvious.
But to answer the question: Batman wins. He's a better planner. That's how he wins battles even when he shouldn't.
What's wrong with voicing my opinion to the thread's topic, better yet the thread's topic has nothing to do with other battles that I'd like to see, not to mention that I didn't say something like Arrow "slaughter-stomps" "curbstomps", etc. So pick on someone else.
(Also I'm the same guy on an alt.)