Because an income tax is far more fair than a sales tax, which would hit the poor far harder as a percentage of their income than those of us who are not poor. Creating more impediments to the success of the poor by punishing them with a regressive sales tax will end up costing us far more in the future than a sales tax will bring in. From the Stiggin' It standpoint I understand that a sales tax SOUNDS better but I don't believe it is.
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Keep in mind that our Legislature isn't exactly filled with pillars of morality. Last time the sales tax idea came up about 15 years ago, my Republican state senator, who also happens to own a business that sells big-ticket items, suggested that a sales tax should be only be applied to the first $5000 of value of item sold thereby effectively giving his wealthy clients a tax break. 
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In addition, I don't buy the idea that all poor people are lazy and deserve to be punished for it. If being poor was so awesome, everybody would be doing it.