I don't think it is much on the popularity of Struts per se, but more the MVC Design Pattern. As we all know, Struts was one of the first web application frameworks that adopted the MVC (Model-View-Controller) concept, allowing for clean separation of presentation and controller aspects within the web tier.

There were other frameworks that mushroomed from the MVC paradigm, such as Apache Tapestry (which didn't gain much popularity, due to performance problems), and JSF.

Nonetheless, Struts has been adopted as the industrial standard for most J2EE software development for quite some time.