Let me preface this by saying that everything in this post is pure speculation. Like all conspiracy theories this could just be the ravings of an utter loon. But hear me out before you rush to judgement. I've spend quite a bit of time pondering the matter and I like to think I'm not a complete idiot, but perhaps that's being a little optimistic ;)
Telltale would have lost a heap of money on the delay, and potentially part of their reputation, so there had to something seriously wrong, seriously game-breaking for them to pull the plug on all the pre-orders. And even more bizarre is that they were ready to release this thing, like really ready. So why would they delay the game until it's completion, rather than just delay the first episode? If the problem was just with the graphics, sound, or action they could have fixed it without needing delay the series until all the episodes were ready. It had to be something that affected the episode structure as a whole.
So what is different about Jurassic Park: The Game? There is the action component, sure, but I don't believe that is what caused the delay. Another big change: this is the first Telltale game in which story choices can affect things that happen in later episodes. Ah, but not quite the first game; in the first Back to the Future episode, Marty has to come up with an alias for himself for 1930's Hill Valley: Harry Callahan, Michael Corleone, etc. People would call you the name you chose, and supposedly in later episodes they would remember it and call you the same name. Now, I say supposedly because this feature didn't seem to work for a lot of people. There were reports that the later episodes would glitch and characters would call Marty the wrong name. I even remember cases where Marty's were being called the wrong name even in the first episode. I believe (speculate) that this feature was partly a test for the upcoming JP game, and if all those complaints in the BttF forums are any indication it wasn't a particularly successful one.
With the first episode of JP steadily approaching, Telltale appeared to have a problem: they weren't sure if the in-game choices would successfully carry over from one episode to the next. In a game where the choices are critical to the story this could be a potential catastrophe. Imagine playing a game where a character dies in the first chapter but is alive in the third without any explanation. So Telltale did the only thing they could do (aside from release a potentially broken game), and that's delay it until they had all the episodes ready so they could make sure the choice system worked and there was no problem with the data carrying over between episodes.
So that's my theory. If you like it or hate it or hate me feel free to comment. Or if you have your own theories on the delay this is the place to share it, and it doesn't necessarily have to be serious. We may never know the real reason behind the delay but there is no reason we can't take a stab at guessing.