Wireframe is loosely based on a much earlier project called pw-mvc, which first emerged around 2012. Pw-mvc was built as the in-house tool for a team of developers working on client projects, and though it's no longer being active developed, it's in fact still in use on a range of sites; from simple one-pagers to complex sites with tens of thousands of pages.
While Wireframe and pw-mvc share many of the same core concepts, one big difference between the two is that whereas pw-mvc boasted a downright primitive implementation (controllers being regular PHP files, etc.) Wireframe proudly takes full advantage of what modern PHP has to offer: classes, traits, namespaces, and all the other goodies.
If you're wondering why the name "Wireframe" and not "pw-mvc 2.0" or something along those lines, the main reason was to avoid unnecessary confusion (let alone controversy) caused by the MVC part of the name.
The goal was never to produce a pure MVC solution, but rather to borrow ideas from the MVC pattern and then implement them in a way that made most sense in the context of ProcessWire. As a name "Wireframe" is more abstract, and describes what the framework provides rather than the software design pattern that it implements.