The producers could include all the stuff in the original game but instead the new races/items are left for the add-on on purpose. Like in Heroes V, they could included all the factions from the beginning instead making a new add-on for each new race. Imagine if Starcraft 2 would ship only with terrans and protoss as playable, and zergs would be left for an expansion pack.

And usually a game has enough gameplay possibilities to offer, so and add-on would make sense only after I completed the game a few times and I feel that I've had enough of it. Even so some add-ons have a bad habit of unbalancing the gameplay, because usually the new features are too overpowered.