Dragon Ball Xenoverse 2 is focusing heavily on DBZ and there is a reason why the original Dragon Ball series is not included. One of the reasons is due to keeping true to canon.
Crunchyroll had a chance to interview Masayuki Hirano recently who is the producer for Dragon Ball Xenoverse 2. He explains why the original Dragon Ball series is not in the game.
Hirano says it’s difficult to add these characters in the game even though he loves the original series. The main problem is that during that time, Goku was only a child and didn’t learn how to fly yet. Not to mention all of the other characters didn’t know how to fly either.
Due to the lack of flying, the development team think it would be difficult to rebuild a new system in order to add these characters in. It will be hard to balance everyone out if some characters can fly and some cannot.
Hirano then said if people have any further questions, they will be setting up a website so that fans can send any feedback their way. Dragon Ball Xenoverse 2 will be supported for a long time as there should be new characters and other things added as DLC.