It's really turned into quite the wait. I'm not gonna complain though - I asked for this after XY and ORAS, after their postgames turned out to be less than what we had hoped for. I distinctly remember saying "I hope Game Freak takes their time with the next game! I don't mind waiting another year if it means a really fantastic experience!" Whoops! But I do wonder what could be causing the delay :o Any game being built in Kalos and with XY's engine has to be relatively easier to implement than anything else they could do. So what gives?
I also find this quite odd. Hoenn had to be built from scratch for ORAS, but it was still finished within a year of XY. If Z is anything like its predecessing third version games, the vast majority of its content will be copypasted from XY, and whatever remains definitely wouldn't take a year and a half to complete, especially considering how fast they made ORAS. Also, Game Freak is a game company after all, and they need projects to fill their time with. They have to be making games at all times, since that's what makes the company go around. It's not like they took half a year off after ORAS; rather they would probably be well underway with their next game before ORAS even hit the shelves. Even if they had started making Z the day after ORAS came out, they would have had it ready for release a long time ago. Whatever the reason is for delaying the release/reveal of Z, time constraints can't possibly be the major factor.
If I were to hazard a guess, it has something to do with Nintendo's next console, which is set to be released within a couple years from now. The 3DS does not have enough left of its life time to cram in another Pokémon generation on the console, even a short one such as Gen V. However, any new Pokémon generation on the so-called NX would still be a couple of years away today, longer than the expected shelf life of Pokémon Z. Rather than releasing Z a year after ORAS, and then having to wait two years for the next game after that, they could be delaying Z for half a year, and then have another year-and-a-half waiting period until Gen VII shows up. They're splitting the long wait in two halves instead of 1/3 and 2/3, or something like that.
The NX was intially expected to be revealed at E3 this year, but apparently hit a delay. It is possible that Z was planned to be released this fall, with the NX and new games set to be released at the end of its projected lifetime (18 months later or so). However, delays happened, and in order to make the post-Z-wait bearably short, Z had to be pushed back a few months, on relatively short notice. This makes sense in the context of the Anime, where episodes are typically finished half a year before they air. By the time the Zygarde episodes were written, everything appeared to be on track, and their airing would tie in neatly with the reveal of Z. However, delays happened, Z had to be delayed, but the episodes were already written and there was no time to make replacements. Hence the reveal of new Zygarde formes long before there was even a game for them to feature in. Now, the NX probably wasn't long delayed, so the expected post-Z-wait is still too short for a new Pokémon generation on the 3DS. Instead, they count on ORAS being solid enough to last us through the long wait for Z, and Z is made to be good enough to last us through the wait for Gen VII, however long that may be.