I think an easier question would be "what makes it accurate?".
I have to agree with Raptor; the gameplay is just most suited to Rome. I didn't like the addition of firearms in M2TW, and I certainly don't care for it in ETW.
Imho a good melee is far more interesting than people firing muskets at each other; they spend 3/4 of the time reloading, ie nothings happening. When I started playing, I just fired a single volley and then charged before they could reload anyway.