Because when taxes are cut the revenue to our government increases.
Even Laffer said this is true only when the top tax rates are high. It does not work when tax rates are low. It's "the Laffer Curve" not the Laffer Straight Line! Most economists have figured out the optimal maximum tax rate for maximal government revenue is about 70%. We are way lower that that now, so lowering tax rates will only lower government revenue and increase the deficit. Laffer Curve (Wikipedia)