Caching has killed the need for SSG's for the most part. You can cache Drupal or Wordpress or pretty much anything with either plugins/modules or via a proxy like Varnish. These things work, generally, by copying the dynamic page and serving up a static page until they detect the page has changed or the user is requesting dynamic functionality (trying to post a comment, log in, etc).
I interpret the groundswell interest in the SSG a testament the CMS does not address all the needs in the content publication pipeline. Some organizations have a focus on content creators. Others on developers. In general, the hacker mindset has an allergy towards ecosystems that remove control. The SSG is about providing a framework for the developer to assert control and regaining flexibility to deliver content independent of coding to a "system".
The CMS is a crutch. It has its place in certain organizations. But it is overkill for many use cases.
In truth, however, we shouldn't be pitting the SSG against the CMS. I believe the next leap of content creation is a mixture of both SSG and classical CMS. Check out the following links:
So you get the best of both worlds.