Among the many reasons long-term capital gains taxes are lower than income taxes is that they are not indexed for inflation over a potentially very long term. If you buy an asset for $250,000 and sell it ten years later for $300,000 then you lost money after adjusting for inflation. Nonetheless, you still have to pay capital gains taxes on the nominal gain of $50,000. You pay taxes on a real loss.
Long-term capital gains have to be much lower than income taxes or no sane investor would make long-term investments. The average rate of return required on risk capital just to break even would be so high that otherwise viable ventures would no longer be viable. Even if you do not understand the return on investment calculus for long-term investment, rest assured that most investors do and will handle their money accordingly.
It is ironic that many of the people that assert most investors are only interested in short-term profits actively campaign to eliminate all possibility of profit on long-term investment. They create the thing they detest. As a more practical matter, long-term capital gains are frequently recaptured as income taxes within two years, after being put to productive use, so there is little lost in minimizing taxes on long-term capital gains in any case. Short-term capital gains -- capital gains with the properties of income -- are already taxed at income rates.