You're right - cost is the reason, but perhaps for reasons you might not thought of. Beginners come in all varieties; experienced programmers learning a new methodology, students, retirees looking for something new to learn, and so on. All of these I've had experience of and provided support for over a number of years on the Beginning series of books. Therefore yes, we chose a tool that was free, had a good pedigree (written by the ASP.NET team), and was small (for easy downloading). This means that everyone who buys the book can have the same experience.

Visual Studio is an excellent tool, but is not only expensive, but also has far more features than most people need (especially beginners), and has a project based system which is confusing for tradition ASP programmers. There are other excellent development tools, as well as other editors without design environments, and we simply had to pick what we thought was the best for the audience. Oh, and we like Web Matrix - that should be taken into consideration too. We all tend to recommend things we like!

So you're not missing anything. I suspect other books have picked Web Matrix for the same reason.