Flat Tax: If they can do it, why can't we?

If lesser developed nations like Montenegro, Albania, and East Timor can implement a low flat tax, why can't we?

Montenegro is moving to a 15% flat tax on personal income that will gradually be lowered to 9% by 2010. Albania's will be 10%. East Timor seeks to set itself up as a free-trade nation (much like Hong Kong and Singapore) with income tax rates between 5-10%.

Update: Russia's politicians reject progressive tax scheme, keeping the current flat tax system in place:

Russia’s flat tax has been remarkably successful. Growth is reasonably strong and tax compliance has improved. Indeed, inflation-adjusted personal income tax revenues have been growing at double-digit rates.