The three reasons I see are speed, security, and workflow. A static site will generally be faster (although with good caching and configuration, you can generally make Wordpress pretty fast). Also nice to not have to worry about any potential WordPress exploits. Finally, since WP entries are stored in a database, the most common way to create and edit them is through the browser. Lots of folks are more productive with the editors they are already using for coding websites.

That said, WordPress successfully powers millions of sites and certainly isn't going away. And since it is dynamic, you get some nice features that would be difficult to replicate with the static site approach. If it works for you, I'd stick with it without worry.

Finally, you can always publish a static clone of a blog created in Wordpress. I've done this before on production sites and it worked well. An extra step, but I got the speed I was after without having to migrate or recreate the site.