The interesting periods for CA to portray are near limitless. The amount of periods that CA can promote successfully to a mass market is frightfully small.

Average Joe is a dumbass.
"ZOMG Samurai, ZOMG, kniggets, ZOMG Romans, ZOMG, Kniggets again, ZOMG muskets" is about as much recognition as could be squeezed out of Average Joe for the previous titles in the series. Anything more obscure then that and marketeers would be having headaches. Personally, I think they might be a smidge lazy not to try. Build it and they will come. It's a risk, and when millions are spend risks are avoided.

Even stuff like the Pelopponessian Wars or Thirty Years' War would be somewhat hard to pitch because not much is known about it by Average Joe. And these are among the most significant and iconic wars in human history! You can forget about a TW game about the Khmer Empire! I'm amazed they even dared tackle the age of Marlborough and Louis XIV. I doubt it would have happened if it weren't for the American War of Independence being included in the sales pitch!

So we re-fight familiar wars at Gettysburg, St.Lo, Waterloo, Cannae or Agincourt for the umpteenth time.