The main point is #2 below, it a double or triple tax, a tax on money that has already been taxed. A higher rate would incentivize more corporate borrowing, which is probably not a good thing.
A quick google returned this:
"The justification for a lower tax rate on capital gains relative to ordinary income is threefold: it is not indexed for inflation, it is a double tax, and a higher rate would encourages present consumption over future consumption, thus discouraging savings.
First, the tax is not adjusted for inflation, so any appreciation of assets is taxed at the nominal instead of the real value. This means investors must pay tax not only on the real return but also on the inflation created by the Federal Reserve.
Second, the capital gains tax is merely part of a long line of federal taxation of the same dollar of income. Wages are first taxed by payroll and personal income taxes, then again by the corporate income tax if one chooses to invest in corporate equities, and then again when those investments pay off in the form of dividends and capital gains. This puts corporations at a disadvantage relative to pass through business entities, whose owners pay personal income tax on distributed profits, instead of taxes on corporate income, capital gains, and dividends. One way corporations mitigate this excessive taxation is through debt rather than equity financing, since interest is deductible. This creates perverse incentives to over leverage, contributing to the boom and bust cycle.
Finally, a capital gains tax, like nearly all of the federal tax code, is a tax on future consumption. Future personal consumption, in the form of savings, is taxed, while present consumption is not. By favoring present over future consumption, savings are discouraged, which decreases future available capital and lowers long term growth."