There is a certain amount of uniformity that is necessary for a Q&A site like MO to work well. The aspects that make good/poor math questions are pretty uniform across major fields. There are already substantial differences between math and statistics, for example, enough to warrant a [separate site](http://stats.stackexchange.com/). I think math ed is similarly substantially different. (Note that there is already a proposal for an [education site](http://area51.stackexchange.com/proposals/50826/education) where math ed might fit better.) This may be disappointing but it appears to be the nature of Q&A sites like MO that guides these divisions.