In the US, the tax code is such that you pay taxes on capital when you've lost capital too, not just on the gains. This doesn't happen with normal wage income hence why it is treated differently. The lower tax rates on capital reflect the reality that you are paying taxes on the losses too.

Tax rates ideally should be flat for all sources of income net of risk, loss, and inflation. To achieve this you either allow deductions for these, which are limited or non-existent in the US, or you lower the tax rates to offset the fact that you can't deduct these.