It is because your income on the capital gain is over $483,900 before you add any other income.
Just like the tax brackets depend on the amount of income tax you pay the same is true with Capital Gains. When you total income exceeds the amounts below you end up being charged a higher tax rate on your Capital Gains.
FILING STATUS 0% RATE15% RATE20% RATE
Single Up to $39,375 $39,376 – $434,550 Over $434,550
Married filing jointly Up to $78,750 $78,751 – $488,850 Over $488,850
Married filing separately Up to $39,375 $39,376 – $244,425 Over $244,425
Head of household Up to $52,750 $52,751 – $461,700 Over $461,700
Link to more information about Capital Gains
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