The foreign tax is carried over because you didn't have enough domestic tax on foreign income in the year it arose to allow all of it to be used. It can only be used in a year that you have foreign income because the foreign tax can only be used to offset US tax on foreign income. So, if you have no foreign income in a given year, you will have no US tax on foreign income that year. So, there would be no US tax on foreign income available to offset the foreign taxes carried over.
The double taxation issue comes in to play because you have foreign tax. The foreign income was taxed in the foreign country and then the same income is taxed again on your US tax return. So, you get a credit against your US tax equal to the foreign tax paid on the same income, otherwise you would be double taxed.
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