Well, the more difficult/specialized the subject, the fewer available teachers. Plus, someone that knows calculus well enough to teach it likely has other job prospects (competitive pay).
You don't need a masters degree in math to teach freshman calculus.
I was taught freshman calc by grad students - with no "education" training whatsoever. Perhaps these "masters" of education are learning the wrong things.
Freshman calculus just isn't that hard, and isn't much beyond algebra. This "gee math is hard" crap is, well, crap. No PhD is necessary. Hell, I taught it to my kids on the kitchen table, and I have no advanced degree and never took a course in education.
Do you think someone needs any education at all in a subject to each it? Do you think someone who has never studied calculus should be able to teach it?