Labor doesn't necessarily generate income. Factory workers are an up-front cost converting one set of resources into another. No income is realized until that second set of resources (i.e. products) are actually sold.

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.