This has been a covered a lot before, but to the extent people aren't aware, new game plus can actually be very, VERY complicated to implement properly in ANY game.
It's not just things like scaling the enemies. It can be things like quests breaking if they're not done in a certain sequence or, for example, a quest depending on the player passing a certain trigger point to continue, which trigger point the player may be able completely to avoid by accident if they have brought over from a previous game a jumping (for example) skill that they would not normally have. It could also be players able to access areas of the game already at a stage when they need not to be accessible, or quest rewards not working properly because the player already has them, etc.
Best result: the quest breaks. Worst result: the entire game breaks. Multiply by 10,000 issues, every single one of which would need to be, first of all, identified, then checked, and then, if necessary, fixed *without breaking anything else in the process*.
They presumably looked into whether it was practically achievable. Clearly, since they have said there will be no NG+, they concluded it was not.