Newt Gingrich explains why in this piece from Human Events:
The federal government would take in about $362 billion more than currently projected over the next ten years if the death tax was eliminated entirely, allowing people to pass the fruits of a lifetime of labor on to their families as they chose. There would be a direct revenue loss but, as the report estimates, U.S. Gross Domestic Product would increase 2.26 percent in a decade just by eliminating the tax. New revenues generated by the economic activity that would result from the elimination of the death tax would be almost twice as much as the revenue derived from it.