It's quite obvious why they removed it. I haven't even had time to buy the game yet and from what I've heard people have be able to get to 2nd, 3rd, even 4th, prestige, just from playing this one map. In order to sell players DLC they need them to still be interested in the game long after its release, and therefore they have to make the leveling up progress relatively slow. Removing nuketown was probably their way of doing that. Although, I have no doubt they'll end up adding it back as people are gonna be royally pissed off.