Doesn't discussion about software development methods belong to Programmers? They actually do have that topic in FAQ.
If it goes beyond software development it is welcomed on Project Management. Actually it is probably welcomed on both sites, Programmers and Project Management, even if it's hard to draw a clear border between software development and project management domains.
Scrum for example bonds both areas pretty tightly so it's hard to clearly address Scrum-related question here or there.
Kanban is tricky as well, since it is used from manufacturing through software development to personal productivity, but I guess vast majority of Kanban-related questions would belong to Programmers, Project Management or both. Also I believe there are more people interested in different approaches to software development/project management, including Kanban, than those interested in different uses of Kanban, so SD/PM sites work as better common denominators.
I don't think we can draw clear lines between topics like software development, project management, let alone narrower ones like agile or Kanban, thus I believe we shouldn't split our efforts into a number of sites and focus on those which are already working.