In essence there is nothing wrong with Wordpress or Drupal. They are the standard CMS platforms of choice. But with that standard comes the fact that they need to be able to be everything for everybody. Which means a bloated framework, which in turn means less than stellar performance, frequent security updates and a bigger chance of using insecure modules or plug-ins.
Another factor of a bloated framework is cost of running the site. nimbly requires minimal server specs to run, keeping energy cost low.
Our goal with nimbly is to provide a CMS for a typical audience. An audience that would like a small sustainable informational website, with an option of a simple blog, portfolio and community. We know this audience exists, because we've been building sites like these for customers like these for years.