My guess is that the deal with Epic wasn't actually permanent for contractual reasons, but it was communicated this way because the devs really liked Epic. They wrote a lengthy post about this on Steam:
"We believe in Epic. Epic is taking a step to fund indie games the same way that AAA is funded -- with deals worked out beforehand, but paying devs rather than suits. They've also publicly stated they're taking 12% of a game's revenue rather than the industry-standard store cut, which is almost triple that figure. Instead of flooding their store with anything, they're specifically curating good games, aiding to our visibility and to gamers' benefit (this also means we spend less time marketing, and more time just making the game). They've been clear with their roadmap, timely with their communication, and human about all our interactions. Overall, they're a company, and a group of people, that we want to work with -- for Airborne Kingdom, and hopefully in the longer term."