It's probably not included at launch simply because it's a lot of work to do for launch, and Frontier would have to choose between delaying the game or Mosasaurus not being there at launch. To have Mosasaurus means to also have its enclosure, feeding mechanics, and a bunch of other stuff for just one animal, and I feel like they would want to include other marine reptiles so it's not just Mosasaurus, and that's more work to add in. Mosasaurus is also gargantuan, much larger than a true Mosasaurus. It and its enclosure would take so much space, and it would all be dedicated to one animal. Could its enclosure be damaged? Could Mosasaurus escape? How would we the players be able to solve the situation? If not, what does Mosasaurus offer gameplay wise?
Also, personally, I don't find Mosasaurus to be that important. Objectively speaking, it was very important, but in terms of what it has contributed to the franchise, it stayed in the background most of the time and the only point it added to the story was when it ate Indominus, which had played a far greater role than Mosasaurus.
But even so, while it may have played a role in the movies, we have to think how practical having Mosasaurus would be in JWE. We gotta think about this from the game development standpoint. Just because it was in the movie, doesn't mean it will fit right in the game. And that's not to say it can't and won't come at a later date. I'd like that, but we must take into factor all that goes into it, because what I've mentioned only scratches the surface.
