As of Piwik 2.0, Piwik has been rewritten to use the latest technologies (PHP 5.3, namespaces, twig, etc.). All plugins available for Piwik 1.x have to be rewritten. Often the changes are minor, but still takes any developer at least a few hours to convert it.
We hope that all plugins developers will take the time to update their plugin to work with Piwik 2!
Consolidating milestones FTW