A theory often has proof supporting it, but I'll bite.
The reason X/Y was so underwhelming was because of various reasons, both due to technical issues but also design choices.
First, they decided to make it easier because people have less time to play games. That was their reasoning, and they were obviously wrong. It's one of the things I hope gets fixed in OR/AS.
Second, they had to not only model, but also animate ~800 models. And that's only the in-battle Pokemon models I'm counting. So add to that all the cities, the routes, the characters, the overworld Pokemon, and you can see that a lot of 3D work had to be done.
Add to that the fact that they wanted to release it the year after B2/W2, and you can kinda see why it was kinda crap. It was a complete game, but only in the sense that this was all they were able to do in the time they were given.