As series producer and Square Enix exec Shinji Hashimoto explained to IGN, director Tetsuya Nomura is working on both games simultaneously, making it tough to deliver them both in a timely fashion.
"It’s a difficult job, as we want each game to be perfect in terms of quality," Hashimoto said. "A lot of people have wondered for the last couple of years if Square Enix has actually been working on Kingdom Hearts III; that’s why we announced it at E3 this year. But please, we need you all to understand that there’s a lot to do."
In other words, even if Final Fantasy XV makes 2014, we ain't seeing Kingdom Hearts III til at least 2015. Of course, that shouldn't really be a problem—a great delayed game is always better than a mediocre rushed one—but it's important we keep our expectations in check.
Now let's play Square Enix Mad Libs:
"Sorry, we must [delay/cancel/add microtransactions to] the latest [Final Fantasy/Kingdom Hearts/Dragon Quest]. Please [understand/be excited/don't leave us]."