Because it was already taxed at the same rate as income when it was earned as profit by the company. Taxing it again when its realized means it was taxed once when the corporation earned it, and then again when the individual stock holder realized it. Now you may think that's how it should, but that issue of double taxation is why capital gains tax is less than income tax.

An alternative is to 'flip' the tax. Tax companies at the rate of capital gains, and investors at their tax bracket 'normal' rate. This also leaves more capital for companies to reinvest.