On the surface, this looks like it's a great idea. However, changing to deltas would entail a great deal of work and typically yield minimal benefit.
The reason is that any changes to the underlying mechanics files for the game system must be propagated up through all the packages that utilize those mechanics. This means that most new releases will result in everything being rebuilt to incorporate the lower-level changes, which in turn means everything still needs to be downloaded anew.
So even if we invested the time to enable deltas, we wouldn't be able to leverage them very often. Compared to many of the other features already on our todo list, this one is unfortunately near the bottom due to its limited benefits in the majority of scenarios.
@Chiefweasel: This mechanism is completely unrelated to licensing and license re-assignments, so there would be zero benefits regarding license re-assignments by supporting deltas.