Simply put, if you cut taxes and spend freely today, you have to raise taxes and cut spending later. If you were a rational taxpayer and you think the government does not have the will power to do the later then you won't believe the former. It might explain why the private markets and the taxpayers don't believe in the current stimulus package.
Here is another take from Naked Capitalism
For a fiscal stimulus (current tax cut or public spending increase) to boost demand, it is necessary that the markets and the public at large believe that sooner or later, measures will be taken to reverse the tax cut or spending increase in present value terms. If markets and the public at large no longer believe that the authorities will assure fiscal sustainability by raising future taxes or cutting future public expenditure by the necessary amounts, they will conclude that the government plans either to permanently monetise the increased amounts of public debt resulting from the fiscal stimulus, or that it will default on its debt obligations. Permanent monetisation of the kind of government deficits anticipated for the next few years in the US and the UK would, sooner or later be highly inflationary. This would raise long-term nominal interest rates and probably give risk to inflation risk premia on public and private debt instruments as well. Default would build default risk premia into sovereign interest rates, and act as a break on demand.
Beacause I believe that neither the US nor the UK authorities have the political credibility to commit themselves to future tax increases and public spending cuts commensurate with the up-front tax cuts and spending increases they are contemplating, I believe that neither the US nor the UK should engage in any significant discretionary cyclical fiscal stimulus, whether through higher public spending (consumption or investment) or through tax cuts or increased transfer payments...