Comparing this DLC to the other major DLCs (e.g. Sword of Islam and Legacy of Rome) is missing the point. The development cycles are completely different. This DLC does not require engine modification, and thus does not require the same developers to work on it, as are working on other major DLCs (such as perhaps a Pagan DLC).
Why release this DLC before an important DLC is finished? Because this DLC is finished, and the others are not. I suspect, that while the event scripters, historical researchers and artists are waiting for the developers to finish important aspects of the other DLCs, they decided to work on something else, so they weren't wasting their time.
Essentially like this; they weren't doing anything anyway, and they decided to create a DLC that would only require skill/access that they had (that is, the scripters and artists).
Thinking that this DLC took any time away from a major DLC is simply not understanding how the development process works. It seems more like there was a vacuum in time, because important design decisions and code changes had to be made in the engine by the programmers and developers, so those creating content simply had to wait. Trust me, this happens. A lot. So it makes a lot of sense for them to do something else.
Usually, you tend not to bring in artists and scripters until late in the game development process, but since they are probably already on the CK2 DLC cycle, it made no sense to take them off, put them on V2 or HoI3 or EU4, because they would be back at CK2 in a jiffy! So rather than wasting their time, they made this.
So stop bickering about time being taken away from 'something important', because it makes you look stupid. I am not saying you are, but it is clear that you are complaining about something that you don't understand how works.
Complain instead, that the scripters and artists decided to create an Aztec invasion DLC rather than something else they could do (this does not include Pagans, Republics, Theocracies, other types of religion/government, any new game mechanics, interface features, etc.; it only includes events and art).