The last Smash balance patch only served to tone down Bayonetta, and that will probably continue to be the last one. That might be at least as intensive than Splatfests, but it's arguably more necessary.
Running Splatfests entails, among other things, writing and localizing text, locking colors, maps, and teams, tracking individual win records and scores, setting up Miiverse to accommodate team submissions, making new art assets, and sending out PR accordingly, all while ensuring that it doesn't interfere with whatever else Nintendo is doing at the time. That's not something that can be set to be automated, unless they took the "cycle through previous Splatfest themes" route, and even then, you'd need some humans checking up scheduling and keeping the servers in line.