The thing about Battle Bots (heh) is that Bots go by waypoints in Races, and don't need to worry about too many compelx actions. They don't even drift. As for Battle mode, there's a lot of mechanics that are used to counterplay eachother, and the one place that could teach you these techniques that the Bonus stages do not... is a well-kept secret area with a taste for something later. And even that isn't the best example of everything Battle mode offers. Making a proper Battle Bot would be more difficult than a race bot, and if there's plans for that later, they had to be shelved wont come into fruition for quite some time.
But I don't think battle bots exist wont exist because battle isn't popular, but rather, battle isn't more popular because there's a lack of bots to practice on. Battle will always be secondary to Racing, since this is a racing game after all, but I'm sure more people will be willing to try it when bots finally exist, and I'm sure KKD knows that too. I'm sure battle bots will happen eventually... but I would not be surprised if it took a few years.