Different people prefer different formats for information. Some people prefer to read the manual from start to finish while others learn better seeing the same concepts in a hands-on tutorial.

From reading the sample of OP's book, it seems far more practical and accessible than the bash manual, so I'm not surprised that a lot of people would read OP's book that have no interest in reading the bash manual cover to cover.

Maybe because they didn't think to?

Because generally a manual is “here is every single detail of this thing”, while a book like this is “here’s an overview of the particularly useful stuff.”

I’ll go to the manual if I’m trying to understand how a particular thing works, or how to do a particular thing, but it’s not as useful to me for feature discovery.