Earlier quoted context omitted.
One of the reasons why it makes sense to have different rules for investment income is investment income tax (at least if you're investing in a corporative stock) is a double tax - the corporation you invest in is taxed on its profit (so your income from it is less than it would be) and then your income from it is taxed again. It kinda makes sense that parts of the double tax would be less than the single one. Of cou…
There's no reason the government wants to promote long-term investment over income-producing labor, is there?
Also, realize that the government does promote both labor and investments, just through different means. Lowering taxes isn't going to affect how a fully employed individual produces labor; they are trading their time for money. Lowering taxes on investment will promote more investments, since the money for those investments have already been taxed, and since the trade of investments is money-now for money-later, taxes have a much more significant influence over the extent to which someone will invest in a business. Someone fully invested in other things (non-stock commodities, etc) might move some of their investments into businesses instead.