A lot of non-fiction is about trying to make a case or prove some thesis. As such, a book may try to show lots of evidence, deal with many potential counter arguments, or demonstrate that different lines of analysis lead to the same conclusion. If you become convinced of the thesis early on a lot of the book can seem redundant.

When I find myself in such a position I usually just stop reading but include the ideas I got out of it in my notes. Later, if I find myself needing the details I skipped, it's easy to go back.