The problem with VAT/GST is once it's introduced it never goes away. In the UK income tax was a temporary levy to pay for the Napoleonic wars and the population is still waiting for it to end.
It's the thin end of the wedge, the tax will start out low and cover a small range of goods, next thing you know it will be 20% on virtually everything. The temptation is simply too much when faced with a shortfall to add 1 or 2% to the rate, or broaden the scope of the tax.