Zope 3 was not backward compatible with Zope 2.x, nor with the impressive ecosystem of Zope 2.x plugins, and for years there was confusion about the direction, momentum, and relative importance of those two parallel projects. Zope 3 never gained any significant traction, because in adopting a "component architecture" they decided to use XML to connect those individual components, and it felt like you spent more time writing XML than Python. IMO this was a strategic mistake.
In 2005 the Django framework was released. In 2006, Ruby on Rails was released. After a couple years of confusion about the Zope roadmap, developers now had multiple options. You couldn't sell management on new Zope 2 apps, Zope 3 wasn't ready for prime time, and Rails was a significantly(!) more productive environment than Zope 2. (Django presumably is, too, but I have no Django experience and so cannot say.)
In 2010, the Zope community renamed "Zope 3" to "bluebream" to clarify their messaging, but that was after six years of ambiguity. Developers moved to other tools and frameworks, and Zope's developer community shrank until it no longer had a critical mass of developer interest.