Why do some professors not recommend any text books for a course?
Reasons why I do not use textbooks: 


Most textbooks in my experience are bad. 
Most students do not read them (probably because they are bad). 
They are expensive. 
They do not align with how I want to teach the course.
I can provide the relevant subset of content myself.


I can't see a reason to use a textbook in my courses other than for supplemental material (even then, I would much rather point them to some free PDFs/slides from various faculty that are online).