I'll answer with another question. How many philosophies on Education consider Mathematics to be one of the pillars of education? Anyone who has studied Education more than cursorily may not have an exact answer but will know that it's the vast majority. I am also a student and a teacher of Physics and I would feel less confidant justifying a separate site for Physics Education and probably consider a more general site for Science Education as more appropriate. I'm undecided on that idea. In the above mentioned philosophies, Science as a whole is just as often considered one of the pillars of education alongside Mathematics.

[PDF: 2010 Mathematics Subject Classification][1]

The sheer vastness of the area of mathematics speaks for itself in terms of the potential for discussion just about mathematics education. Click on the above link if you're not sure just how broad and deep mathematics has become.

Of course there are overlaps in the practice of education across all subjects, but that in itself is not an argument against specialisation. Solving physics problems and applied maths problems has significant areas of overlap but no-one is questioning the logic of the separate existences of Math.SE and Physics.SE - are they?

  [1]: http://www.ams.org/mathscinet/msc/pdfs/classifications2010.pdf