If states decided to foot the bill for free nursery education, more and more stay-at-home parents would be able to enter the workforce. Many of these parents want to work, but cannot find a job to offset daycare costs. By eliminating that cost, they can enter the workforce, earn a wage, pay taxes, and be more productive.
There is no evidence that pre school benefits those students who attend pre school versus those who do not. As such, it would be completely unjust and immoral to force other people to pay for some one else's nursery school. If a program does not work, other taxpayers should not have to foot the bill for it.