I know many of you might be sick of me talking about Digimon: Next Order but we got an official answer regarding the reasoning of no PS Vita release of the title.
According to this interview, and this is the only official reasoning we will ever get, the improved PS4 version would be too different from the Vita version and implementing all the new elements would be too difficult in the Vita version.
For example: the PS4 version comes with extra 30 story quests. These aren’t just extra missions that you play after beating the main campaign. They are actually part of the main campaign so the whole story structure would’ve changed. The way the translator put it, it sounds to be that the PS4 version will be similar to how Digimon World Re:Digitize Decode changed and added some story elements to the main story that are not simply stuck to the end of the game. And all of these improvements also remained 3DS exclusive.
So the PS4 version is basically a next 0rder decode. Although it has to be said that the difference between Re:digitize and re:digitize decode was bigger because the UMD’s had very limited storage capacity. Some of Decode’s content was meant for the original release but was cut.
So the reasoning is that Bandai-Namco does not want to port the new content produced for the PS4 release into the original PS Vita title. This is an official comment from Bandai-Namco and likely the last PR release for this game until a later date, so at the very least, things are finally cleared up.
Source: Game Reactor