That's because it's not a proposal about existing arguments. Signal processing is about theory of that, not matlab implementations. Otherwise stack exchange sites like Mathematica should be removed for the same reason.
It's about on how to use the program, how to personalize, it, how to debug scripts in, how to implement algorithms in scripts.
the site is not about "How can I define a Kalman Filter", but it's "I want to implement this Kalman filter".
Many times it's related to programming m files, and maybe the question can be included in Stack Overflow, as now, but there can be questions about toolboxes, documentation, Simulink diagrams and other stuff that are not related to programming.
So in my opinion this site has the same reason to live as Mathematica site do (But also Joomla, Drupal and so on).