I am not that knowledgeable in US education and policies, but
there is/was some "no child left behind" thing going on and I may be completely off, but my impression was it has something to do with all this.
My (again, possibly completely wrong) understanding of it, that the end effect was "let's pretend we don't have dumb students" instead of "let's help the bright students".
If someone can shed more light on it I'd appreciate that a lot.
I may not be much better suited to answer your question than you, but my understanding goes more like: the intent was "let's use standardized testing to identify and then help weak students and to support schools that yield good overall students (according to the test results)" and the result was schools trying to game the tests to maintain/gain funding.
If someone can shed more light on it I'd appreciate that a lot.