I think it's more about license and Money. Sakaurai needs agreements from:
-Akira Toriyama (creator of the character),
-Bandai Namco (owner of the rights of making Dragonball games),
-Funimations (have the rights of the Animations)
-Masako Nozawa (japanese voice of Goku).
The problem is that Smash isn't a dedicated Dragonball game and getting the 4 Ok's take a long time. Unless Sakurai can spend so much money and gets the agreements immediately.