Codraroll I'll give you this one tell us why it'd be absolutely horrible business-wise for game freak to only release games every two years
Well... strictly speaking, it's only bad for business in the sense that they have the means to release a game quickly (updating the Kalos games in a year would be a trivial matter considering they built Hoenn from scratch in a similar time frame), there is obviously consumer demand for games so often (ever since BW, we've got one game every year, and ever since ORAS, the Internet has been screaming for Z), and that the studio's primary expenses - staff wages - remains relatively unchanged regardless if they cut a game out of schedule or not. By not releasing games, they're practically turning down money.
Thing is, the Pokémon games have high replayability, but many people do not want to replay them. After investing dozens of hours raising your Pokémon, collecting items and filling the Poké Dex, you do not want to throw it all away, which is required to start a new game. Trading it over to another game is a huge hassle, and stuff like Secret Bases, Maison trophies or Pokédex entries cannot be traded over. You're still undoing hours of work by restarting your game. Still, many of us play Pokémon for the progression alone, starting as an inexperienced kid from a sleepy village building progress until we're the champion of the region. That feeling of progression is an essential part of the Pokémon experience, but it can only be experienced once in a playthrough. The demand for that progression, along with the negative impacts of starting over and the fact that exploration is usually way more fun the first time, means that fans will crave
new games to play instead of starting over their old ones. That's why we're so worked up over Z not being released, and why many of us would buy it without question even though it's 90 % the same as the games we've already played to the point of boredom. Financially, it makes sense to release a game every year, which is just enough time for fans to complete the old game, get tired of the post-game (which is usually stripped of features so the next game will look even better by comparison) and engage in hefty speculation up until the release of the new game. Take too long, and fans will complete the games, wonder "now what?" and maybe engage in some other franchise which delivers a similar thrill. When the games eventually do get announced, their interest might have waned to the point that they don't bother to buy it, or at least puts it on hold for a while.
That being said, there is an extra element to it too. Namely, Game Freak's dependence on Nintendo's handheld consoles. If the 3DS's projected remaining life span is too short to accomodate another Pokémon generation, they have to somehow keep Gen VI relevant all the way until the next console is released and Gen VII can take over. Release the last Gen VI game too soon, and the waiting time for the next game might be so long as to risk pushing Pokémon out of relevance. To shorten the post-gen-VI-gap, it makes sense that the last Gen VI game is postponed, leaving another gap in the middle of the generation instead of a huge one after it. It is a pet theory of mine that the pre-Z gap was not originally planned to be that long, that ORAS was not designed with such longevity in mind (the lack of a Battle Frontier was a conscious choice, seeing as they left a model of it in-game) and this is why we're so impatient for Z. ORAS could not keep us thrilled for long enough to cover the gap; it was designed to be fun for only as long as it would take Game Freak to release the next game.
That, or they're testing and seeing whether the spin-off games perform better in the absence of a new main series title on the market. With a shorter and less intensive development cycle, and being comparatively independent on the "generation" format, spin-offs put less strain on the studio while also being easier to plan for. Main series games come in large, unwieldy, costly chunks (generations) that have to be developed for the same console and released over several years, spin-offs are stand-alone and can be made platform-independent if need be. We've been
bombarded with spin-offs this past year, most of which have failed to capture the thrill of a main series game. They aren't as engaging as a full-fledged Pokémon adventure, despite having Pokémon in them. The brand alone isn't synonymous with a fun and engaging gameplay experience, and it seems obvious Pokémon will have to rely on the main series games in the future too. Hopefully, this two-year gap is either a mistake due to unforeseen circumstances, or a one-off failed experiment.
(P.S.: I left out issues already covered in the "longevity of Pokémon" threads such as content crunch or platform migrations. They're not that relevant to this issue).