The 575 M6 is indeed a Great mike.
My guess as with many Excellent items is that it was discontinued simply because of it having been made so long (becoming dated in its looks) and or that it became to expensive to continue making for the dollar amount it was selling for.
I agree with you that makes good business sense. If Astatic decides to reintroduce the 575 M6 I will one of the first to buy two of them.