ASP.NET Webforms, first released in 2002, has a been a powerful framework for developing enterprise-level applications.
When you have the resources, running a typical Microsoft stack is a good idea when you expect your application to get a lot of traffic. While there are a lot of trendier ways to develop a web application, such as Ruby on Rails or Drupal, ASP.NET sure does have it's benefit.
A recent post on a Harvard Law Blog, Ruby on Rails and the Importance of Being Stupid higlights the biggest reasons why ASP.NET has historically been a good choice - performance. Generally, running the Microsoft stack means that you will not have a to add a second server long after you become popular.
Now, developers and web site owners have a second reason to choose ASP.NET - ease of development with MVC. While some of the modularity of Webforms might be lost, as well as some of the quick-start time on internal applications, MVC Is a great choice for cutting-edge agile development practices. MVC has been used for decades and is extremely popular in the competing Java world, and developers all around are enjoying using it with the ever popular ASP.NET