From CKAN's webpage:
> Strong integration with third-party CMS’s like Drupal and WordPress
There was no explicit reason to switch front-end sites...But think about it...creating Data.gov was a large endeavor, but it's certainly not a mission critical one. Just a couple years after building that initiative from scratch, they throw out Drupal for WordPress instead of using their existing Drupal installation and installing CKAN modules. How does that not reflect badly on Drupal, which bills itself as a much more powerful, flexible CMS than your standard blog system?
I used Drupal back in the 5.x days and thought it was pretty swell, but that was when I was a rookie developer who had almost no real full-stack dev experience. Now I can't think of a single use case in which Drupal's purported feature set is needed that would not be better fulfilled by just going to Django/Rails. The hackery that seemingly has to be done to meet clients' perceived needs almost eliminates whatever advantage Drupal in terms of a community of knowledgable developers to hire from. And I would question whether there are really more Drupal-only developers than Rails-only developers (certainly, there are way more Wordpress developers).
However, there used to be more U.S. Drupal sites, including Data.gov...there was even some hubabaloo about them releasing the source code:
http://www.whitehouse.gov/blog/2011/12/05/datagov-goes-globa...
https://groups.drupal.org/node/194808
Viewing the source code of the current data.gov (view-source:http://www.data.gov/), it looks like the moved it to WordPress. That's a pretty damning migration.
Edit:
The current repo for Data.gov can be found on Github - https://github.com/GSA/data.gov/
Here's the shit-show that was apparently Data.gov's Drupal installation: https://github.com/opengovplatform/ogpl-d7
And quite humorously, it seems to be something that we're exporting to other governments, such as India https://github.com/opengovplatform/opengovplatform-beta