If there were left out by design it was a bad design decision. But, CO never told anything about airports, sea ports, trains etc. - these things simply never were in the game from the beginning. So it's pure speculation why these things were left out. The same goes for landmarks, other architecture styles, bridges, parks etc etc.
For me one part of the additional content is not in the game for time restraints because CiM2 was rushed out when Simcity 5 was released - could be coincidence, could be planned. The game was released super early, and clearly some things were not finished, not to mention polished.
And, another part was not in the game because CO want sell it as a DLC. Or, even better, to give us a present in one of the following patches ;-) As it looks in patch 1.22 there is already designated some more content.
It is open secret that CIM2 was rushed after failure of SimCity 5. What is why we have free "pathes" with new features. However those features are not so new, they were already planed long time ago but because CIM2 was rushed we do not have them from the beginning. So after all those patches are not exactly patches but finishing touches for the game. Many agree that what we see now is an open beta CIM2 for which we have payed. Personally I like this way as I can see the game develop before and it is better for CO as they can have more feedback. Of course CO gets some bad feedback because they say they released the "full game" and dissatisfied buyers get mad because for them it seems that the game is not finished and I have to agree that it is missing few key elements.
All in all reading my post it seems that I'm very angry but in fact I'm very happy that CIM2 was released ahead of time and we got to taste the game and see how it changes ( everyone remembers those nasty buses turning everything to suburbs) CO did a great job and they are still doing a great job releasing "patches" quickly. One thing I am afraid that this "free new features" thing will soon end as they finish to include everything they planed from the beginning and will be replaced by "pay for every new thing DLC"...