Single Player would be too much of a hassle. The ESF engine isn't designed as such to make single player battles too cinematic. The game really shines when you're going back with at least four or five others, and explosions and transformations are happening all over the place. You could do it, but it would need a full setup worth of in-game cinematics and stuff. I'm confident the ESF team COULD do it, whether they want to spend all that time, is another matter.
You've also gotta remember that single-player would mean you'd have to do all the show battles, meaning newly made and animated characters, and lots of them. That's a ton of work. Technically, they'd have to make:
Nappa
Raditz
King Kaioh
Kami
Yamcha
Tien
Chaotzu
c.c I mean... those are the ones from the Saiyan saga that I remember off the top of my head. That's a lot of new models, along with new animations and another set of attacks.