Thorburne
Centurion
The only reason I can think of is fear of too many wonders in the ancient world. Arena or Amphitheater would both work as ideas.
I would prefer the Arena or Ampitheater being the actual buildings and the Colosseum be the wonder. After all, there is only one Colosseum in the world. And, unlike many other ancient wonders, it still exists today. Sure, it is not in pristine condition, but enough to be recognizable as to what it originally was. Plus, it was an Engineering achievement in its day. To me, that qualifies for a Wonder.