Here are some possible reasons for Duncan's position on this issue...
1. He equates "good teaching" with test scores. Teaching students to get higher scores on tests does not require a masters degree. The learning doesn't last...doesn't mean anything beyond the test taking, but that's where he is...tests are the only things that matter.
2. He realizes that if masters degrees in education meant something then his own qualifications would be suspect. He has no educational training. He has never taught in a public school...has never worked in a public school...has never even attended a public school. It's in his best interest to imply that teachers with masters degrees don't know any more than he does with his bachelors degree in Sociology.
3. Ditto number two, above, for his friend and ally in the destruction of America's Public Schools, Bill Gates (aka, the college drop out).
4. If masters degrees in Education were meaningful then Teach For America, which puts non-educators into classrooms where the most experienced, best trained teachers are needed, would not be as good as having education school graduates teaching. Duncan has emotionally invested himself in TFA. He doesn't want to admit that experienced teachers with training are better than college graduates with 5 weeks of training.
5. If teachers with years of experience and advanced degrees are not any better than beginning teachers, then the higher salaries paid to teachers with experience and advanced degrees can be saved. The price of education is too high. The truth is that children in the United States are not valued. We're not interested in how we can improve their learning...just how we can save money. Duncan is playing to the Americans who want services but don't want to pay for it.
The Secretary of Education as well as the President and most US Politicians still won't admit the obvious truth. The fault lies with the system that keeps 20% of our children living in poverty. They blame the public schools, teachers and teachers unions for their own failing.