Why do textbooks often include the solutions to odd or even numbered problems but not both? I've just gotten a textbook (physics) for next year and I noticed that, like many other textbooks I've had, it contains solutions to only odd-numbered questions.

From my experience, this is typical, at least for texts at the undergraduate level. 

If anyone is wondering, the text is "Fundamentals of Physics" by Halliday &amp; Resnick 10th edition. 

My question is, why do textbooks often include the solutions to odd or even numbered problems but not both? In my case, I don't think space is the answer because the answers section only takes up 7 pages.
This allowance is a custom to allow instructors to give homework where the solutions to some questions were not provided directly to the student (at least not in the book - this was from a time where searching for solutions to homework was not so easy outside of personal social connections).

If an instructor just wants students to work on problems where the students can easily refer to sample solutions at the back of the book, the instructor can just assign "problems 1-7, odds only". If they want to assign only no-solution problems, they can assign "evens only". If they want to give a mixture to try to encourage students to mix up their solving strategies, they can assign both. To go farther, putting them at the back of the book was another way to try to make it take a little more effort to look for the solution, to encourage the students to try to solve it themselves rather than immediately looking at the solution.

Finally, it is a custom that the problems tend to go from easier to harder, with some texts making the highest numbered questions of a chapter require more knowledge or skills than is actually provided in the accompanying chapter. 

As you can imagine, this isn't the only system of designing a textbook that would support these uses, but it just became a very popular and simple way to do it - so you can generally expect to see it in many of the textbooks you'll encounter.