Single taxpayer, standard deduction; sole income item is securities capital gain of 36,000. Cap Gain Worksheet has 0 tax as does Form 1040. Why does return show 0 tax?

Capital gains tax rates are often lower than regular tax rates, and long-term capital gains rates can be as low as 0% for those in the 10 and 15% tax brackets.  So, as a single taxpayer with that as your only income item, you would qualify for this preferential rate treatment of 0% assuming it was a long-term capital gain.