There are two possible explanations that come to my mind as reasons for that decisions to go away from a single-type fleet:
- They went the AA way (of negotiating), meaning they negotiated with both Airbus and Boeing until they got a deal from both they could not resist to close them both.
- Their anticipated growth is too large that one of the OEM's could deliver aircraft fast enough to satisfy that growth.
The next interesting question will be the engine choice for the A320neo. Although the selection of the LEAP-1A to complement the LEAP-1B on the MAX-8 fleet would be the most logical choice, they maybe also go the AA way and buy from both engine manufacturers, like AA did it recently for their A319 (CFM56) and A321 (V2500). So the PW1100G could come into play - the factsheet that Norwegian provided with their press release shows the fan diameter of the PW1100G (81"), but that does not necessarily say anything...