As mentioned before there are a few reasons as to why:
1) Strain on the current gen systems. The developers for all the games that are coming out now are now working with engines specifically for the next gen consoles. That means since the current gen doesnt have the upgraded engine, that would put too much strain on the system and it wouldnt be able to handle it without the company making the map on the current gen engine. But the reason for them not doing that is the next point.
2) Transitioning from current gen to next gen. They are trying to make the transition to next gen. That means that they are going to stop making games and DLC for current gen systems and only focus on the next gen because not only does that take less time and money but it also helps them focus on only 3 systems rather than stressing out over 5 ( i am including the 360, PS3, XB1, PS4 and PC with that). This process has been done before with the original Xbox and PS1 when switching over to the 360 and PS2.
So combining the fact that it would take time to make it compatible with the current gens engines and the fact that they dont want to do that since they are trying to push the next gen out for not only more money but also less stress, that is more than likely why Nuketown wont be coming to the 360 and PS3.
That is the way it works.
