A320-200 vs A20N A320 251N

Why do I have to select the A20N A320 251N and not the A320-200 to use Inbuilds A320NEO? I had to ask in many forums until someone told me that you have to use the first one. These are questions that I don’t understand.

The A20N has the New Engine Option, the A320-200 does not. The performance of the two varies.