The footer exists to expose sites that are part of the Stack Exchange network of sites.
You may note that the sites on in are sites that have gone out of public beta - sites that have graduated/launched (meaning that the community team believes they are self sustaining).
Not only is this footer ridiculously large already, but adding more sites, and sites that are outside of the network to it goes against its purpose.
That, before even deciding which sites to add. Do we add sites that directly compete with Stack Exchange sites? Do we decide by volume? What volume? How do we organize them? How do we distinguish between sites in the network and those outside? Or do we distinguish them at all? How many to add? 10? 100? All of them?
People who are looking for Q&A sites in specific topics are not likely to be using the footer anyways - the are much more likely to be using a search engine.