How come nRF51822 evaluation kit isn't recommended?
"Not Recommended for New Designs" from a semi manufacturer usually means that they're planning on phasing that device out sometime in the near future - so if you do use you might find it difficult (or expensive) to purchase.  

In this case, since we're looking at a development board, we can see that the recommended replacement supports the IC from the old board as well as another one.  

They're probably trying to reduce the number of different boards they have to take care of and so they're phasing out the board which only supports one IC in favor of the board which supports both.