Dumbing down is the answer, teachers don't want to be judged by their students lack of performance so if it's easy enough everyone will pass.
In almost all cases, teachers' performance is not judged by student grades, it's judged off of one day of kids filling in bubbles. It's all about the test scores.
The reason most teachers don't want to be "judged" by student performance, is because they understand it is impossible to quantify how well they are doing their job, there are just too many factors.
Most teachers hate common core, but no one, from either political party really wants to hear from the people actually doing the job. It was no child left behind, now it's common core, in a few years it will be some other BS.