Since you mention Joy of Clojure, it's worth a response.
First, I would love to include exercises, however two factors conspire(d) against us. First, some of our favorite C.S./programming books have amazing exercises, so we respect the art of creating interesting and relevant supplementary material. It is no small task to design a book that flows, teaches the right lessons and also provides a set of exercises. If we were to include exercises we would think very hard about what to include and how it fits into the book as a whole.
Second, the schedule for our book was quite tight given that we both have families and full-time jobs. We could write the book in a satisfactory way on time, or we could include exercises. We chose to write the best book possible given our time and ability (or lack thereof). We actually did create some exercises, but felt it was not worth including a partial set. Maybe next time. However, all of the same considerations would apply.
As you'll notice, our exclusion had nothing to do with page count.
: I should say, "for us" since others seem to do it easily.