StackExchange is a network of many, many communities. Usually each within a specific topic. That does not mean a topic may only fit one community (though it should not be cross-posted but migrated). I can think of many questions that fit Aviation and Travel. Just like this, there are many questions that fit StackOverflow and WordPres.
Why is there a WordPress StackExchange? Probably because it was proposed in Area51, got through the process and graduated. You can read about the full process in their FAQ:
#Once I propose a site, what happens next?
Once I propose a site, what happens next?
A site goes through several phases before it launches to the public. Each stage is designed to improve the site and build up momentum and support.
- Interested parties propose and discuss sample questions to define what the site is — and is not — about.
- Users are asked to commit to participate in the site to assure that the site will have enough participation — we don't want to create ghost towns.
- The site is launched for a beta period to seed it with questions, develop the FAQ, appoint temporary moderators, and refine its design.
- If a site reaches critical mass, it becomes a full member of the Stack Exchange Network.
If a proposal loses momentum, it may be re-evaluated or merged with similar proposals. Proposals with insufficient activity are subject to removal. Deleted proposals can be re-proposed anew by users with renewed energy and, hopefully, better resources to bring it to commitment.
