Why are textbook authors often not the most famous/cited researchers?
You have already mentioned most of the things that are going on. 


Someone who is good at doing research is not necessarily someone who is good at writing textbooks. These are different skills.
There are only 24 hours in a day, and an hour spent on writing textbook is an hour not spent on research. A person becomes a top-researcher by being strategic in the way (s)he manages her or his time. That often means not writing textbooks (see point below).
The "value" of a book differs a lot from (sub-(sub-))discipline to (sub-(sub-))discipline. Some value books (but even those not necessarily textbooks), and in some writing a book is basically a waste of time.