"I guess you're referring to the favorable tax rates for capital gains.
Those rates are favorable for two reasons: (1) The money invested to produce those capital gains has already been taxed once, so you're really talking about taxing it twice; (2) Over longer terms, most of the returns from capital gains are due to inflation."
1) Why does it matter how many times the money is taxed? Money is taxed many many times as you follow its flow from raw resource to waste matter. Lets suppose you take the interest from an investment and buy a car with it. Now the money was taxed first as regular income (assuming we are not tax-avoiders), second as capital gains, third as goods & services or VAT or your local equivalent. The end result of reduced rates of captial gains tax is that those who are rich already (have the most money to invest) enjoy a reduced tax rate on total income than those who are not, thus increasing the rich-poor divide.
2) While that may be true (I'm not in finance) average salary over the long term also tends to track inflation thus again the lower tax rate give the rich who can invest an advantage over those who are not.
Finally why should set up tax incentives for companies not to invest in growing and generating more jobs? Currently many countries are giving tax-payer dollars to corporations to try and get them to invest in growing and generating more jobs. Even if for some reason we decide it would be good to discourage companies from investing in themselves and growing, why should accept a set-up were that penalty is paid out to the richest members of society?