I'd like to echo what Dave has already posted, and add a little bit more. First of all we wanted to include as many people as possible and Web Matrix being free was a big plus in its favour. However it doesn't just stop there, and it isn't just another "Personal Web Server".

Without giving too much away, the Web Matrix approach is one you might be getting more familiar with in future releases of VS.NET. When MS give away tools for free, while they are being generous, they also derive some benefit themselves by being able to test new innovations and gauge reaction to new ideas in a far wider environment than just their beta-testers. I'd imagine that there will be aspects of Web Matrix that will find themselves in the next version of VS.NET.

Also on top of this we wanted the book to be all about ASP.NET. If we'd added VS.NET coverage, it would have required several more chapters. We considered it, and we also considered Notepad, but Web Matrix seemed like a good compromise of useful features, but also not getting in the way and keeping the book length below a thousand pages.

Lastly IIS doesn't work on XP Home edition, while Web Matrix plus Web Server do. We intuited that a lot of users would be learning at home, and it would be quite likely that this is the operating system of choice, so once again we strove to include as many possible people in our readership base.

I hope this answers your question,
Chris Ullman