Is your TOTAL income INCLUDING the long term capital gains under $30,000? If not, then it is because the capital gains are increasing your income to be above the level for your filing status, to not be taxed on capital gains. For example, if your W-2 income is $29,000 and your capital gains is $40,000, your total income would be $69,000 so if you are single, your capital gains would be taxed at 15%.
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