We did ultimately see a "Captain's Ready Room" on the 1701. Kirk, Spock and McCoy went there to have an exposition-fest during TMP. It happens to be at the aft of Deck 2.
No, that's not a "ready room". That's the officer's lounge, and it's on the refit, not the original TOS ship [http://en.memory-alpha.org/wiki/Cons...cers.27_lounge
Very true, but Kirk did have a "Office" connected to his quarters that we saw him use like a Ready Room in both TMP and TWOK!
Just like Pike had a cabin with a Ready Room like location and features in "The Cage"
I do agree that the production, real-world explanation is that the studio made Enterprise way too much like Next Generation, DS9 & Voyager.
But an in-universe explanation may be that the original Connie design had a combined Ready Room/Captain's Quarters directly under the bridge. But later the Captain's quarters were moved down into the saucer - maybe because that space was needed - and there was no room left for a Ready Room. (And in TOS and in the follow-up movies, Kirk still had a small office space in his quarters...they just were away from the Bridge.)
Now, big question is why, in-universe, doesn't the nuKirk have a Ready Room - where there *is* plenty of space for one!? (I'm gonna say that he probably does, we just never see it, as nuTrek is a less "talky" version of Trek...more action-oriented, less time to sit around and have a chat. That, and they seem to have Transwarp or Slipstream drive, as fast as they get around the Galaxy...that, and turbo-turbolifts that can get you ANYWHERE on the ship in *seconds*...)