ESF is pretty much a multiplayer experience, similar to retail games like Quake 3 Arena and Enemy Territory. I personally feel that ESF remains enjoyable due to the unpredictability of opponents, it's not always possible to read people.
Whilst I'm sure everyone would be happy with a singleplayer mode, asides from the team (it'd be a lot of work), most people would only play it once or twice at most, leaving it as a tacked on novelty. Due to the large number of options in ESF, bots get awfully confused and need to be told what to do, making them predictable. Of course it's possible to up their reaction speed and fluidity but it makes it akin to fighting an aimbot. Which from experience is no fun at all.
Again, I'd like to mention the workload. Reliving saga's requires models, maps, dialogue and coding. Along witha lot of time. Characters who didn't appear in ESF would either have to be made from scratch or omitted. Imagine jumping straight into a fight with Vegeta, only to have your next task fighting the Ginyu Force. Another issue is that most "levels" would either be protecting brain-dead bots, getting kills or reaching a certain PL. Which could get boring a few levels in.
On paper it's a nice idea, the main problem is that it'd require far too much work to be worth it.
They might fall apart and all that crap. So... 
