Because capital gains are NOT indexed for inflation. If you buy a stock and hold on to it for 15 years, and if you are lucky, it doubles, you are taxed on 100% of your original cost. However if inflation has gone up 50% during the 15 years, you do not have a gain in buying power of 100%.
The economy needs capital to expand. If you remove the incentive for risk takers it will make stocks less attractive, drop their prices and thus further impede capital requirements of corporations since issuing new shares is the best method of raising capital for any corporation.
So then, it seems to me that the fair thing to do would be to index capital gains for inflation and then tax them as a part of regular income.