Drupal has made huge inroads in running US government websites in the last few years. The White House, Departments of Commerce and Energy, and a bulk of the House of Representatives (most on one install, I believe) are all running Drupal, to name a few. As a former government contractor, we ended up selecting Drupal to run one of our sites for a couple of reasons.
First, Drupal can run using SQL Server on the back end. I know that sounds horrible, but for us, spinning up MySQL/Postgres was not really an option, and the .Net management systems were either too expensive or a joke in terms of features. So we got the community of Drupal on the hardware we already ran.
Second, its taxonomy features are really unparalleled, at least from what I have seen. Coupled with Views, you can create a page out of just about any combination of vocabularies. We used it to show a category of content, and then let users filter based on keywords. Think categories and tags from WordPress, but on steroids.
Now, all that being said, I hated developing in Drupal. I was able to achieve the goals for the site, including letting non-developers handle content updates with minimal support. But getting there was the most excruciating three months of my career. People aren't kidding when they say the learning curve is enormous.
Overall, I am happy to see Drupal making progress, and think it is fantastic in handling certain types of websites. But at the end of the day, I hope I never have to build another Drupal website again.