As usual, it boils down to greed

Vouchers are adamantly opposed by the teacher unions, which spent millions persuading Utah voters last week to repeal a voucher law passed by the legislature. No one can say for sure how much vouchers would improve education. But they are "forces of competition," as Greenspan puts it, which we're almost entirely prevented from harnessing because of the power of teacher unions -- the power, more specifically, that they wield in the Democratic Party. ...

The teacher unions are an incredibly important source of money and volunteers for the Democratic Party -- about one in 10 delegates at recent Democratic national conventions have been teacher union members or their spouses. When they snap their fingers, the Democrats jump. Vouchers threaten to dry up dues money, and that is that. -- Michael Barone, Townhall.com, 11/12/07