Visual Basic has slowly evolved over the years, incorporating a variety of features and moving itself into the world of "enterprise" development, but everything was being built on top of an already existing foundation. This is not unusual; most development tools progress in this way, but it has the unfortunate side effect of garbage accumulation. New versions of the tool try to stay compatible with, and therefore have to keep all the less-than-perfect aspects of, the previous versions. To rewrite the language from scratch is almost unthinkable. The work required would be enormous and breaking compatibility with the user's existing code is bound to make you unpopular. The benefit of such a move would be a completely clean and new implementation that can keep the good and throw away the bad parts of the existing language.
That is exactly what Microsoft has done in the move from Visual Basic 6.0 to Visual Basic .NET. They have rewritten the language to create a clean version that does away with the garbage built up over a decade of successive language improvements. That means a hard learning curve for people who were experienced in the previous version of the language, but the end result is worth the effort. This radical shift makes this a great time to get up to speed on Visual Basic .NET, as the concepts taught in this book will have a much longer lifespan than material covering the previous version.
There are many benefits to this change, all of which were motivators for this decision, but the most significant motivation was the need to conform to the new .NET environment. As a necessary bit of background before you can jump into the specific changes that occurred between Visual Basic 6 and Visual Basic .NET, you'll learn more about .NET, including what it is and how Visual Basic fits into it.