The main reason important reforms don't get made in American K-12 education may be termed the Chicken Little Syndrome: the assertion that the sky will surely fall down if this change is made or, more temperately, the suggestion that the sky MIGHT collapse but we can't be sure so let's not take chances.
To watch this syndrome on display, observe the school establishment's reaction to vouchers: we don't know whether they'll work and we're not sure what will happen, so we daren't take the risk. Or the response to "charter states" and other forms of funding flexibility: we can't be sure what innovations those squirrelly states might try so we'd best not gamble. Or "alternative certification" of teachers. And so forth.
Mostly, this is the characteristic response of timid people and organizations with deep vested interests in the status quo. They fear change or believe it would adversely affect them. Their method of fending it off is to emulate Chicken Little, warning that the heavens will crash down upon innocent children if any such innovation is introduced.
Sometimes, though, we have actual experience to draw upon in predicting the likely outcome of a course of action. Sometimes Chicken Little's raindrop...