While I sympathise with Your complaint - that is, lacking a manual which introduces the newest features - I also have my opinion on why this kind of material wasn't provided.
A. The game is quite exhaustively documented, with its tooltips - which are the most immediate form of information. Hardly a substitute for the verbose prose, featured in a manual ... but the quickest and most immediate help, in my view. Certain dynamics are nonetheless not described, within tooltips, as they elude the context of the many buttons and labels. There always is a lot of effort being put into describing events and concepts, with these popups.
B.
Given the continuous nature of development, which assumes that not just bugs are being solved but also game balance is being looked into - it's hard to, at least initially, draft a manual.
How to exhaustively write about, say, the effect certain "titles" ("Legate", "Magistrate", "Consort", "Pro Consul", ...) have on characters - if those are bound to change from one release to another.
If You anticipate an immutable game, with but bugs to be taken care of, You can certainly expect a manual to be a speedier undertaking. It usually is not the case with Paradox titles, which patches do not just address bugs, but alter some game mechanisms - resulting in manuals becoming obsolete rather soon.
The size of this forum mirrors the need to continually update and explain information on certain games, as well as propose new directions.
Regardless of these points, Your argument remains sound: a manual would surely help.
I expect a wealth of information to be shaped up pretty soon - that will contribute to getting You up to date.