This מאמר explains the idea of the 3 בריתות (covenants) made with the world: 1) נח; לטבע. 2) אברהם; לתורה ומצוות. 3) משה; לתשובה. The covenant with משה is for תשובה; which means all Jews are (at least potentially) good. פרה אדומה is תשובה and therefore it was done outside of the camp because it elevates even that.
