Liberals do you know why they tax capital gains at a lower rate?

Judging by the answers so far, apparently not.

Because this was set in place by a republican official. As well, capital gains are not always taxable. 

Because whiners whine, that is why. I know why capital gains should be taxed as income, because it is income.

Because rich people give money to politicians.

I'll assume you're talking about long term capital gains.

To encourage investment, but this misses a larger point. The richest 10% of Americans make the majority of their money through investments which are taxed at the lower capital gains rate. Meanwhile, the bottom 90% of Americans make most of their money through earned income which is taxed at the higher income tax rate. So we're burdening the poor more than we're burdening the rich.

There's a happy middle ground. We could cap the amount of income subject to preferential capital gains treatment - say around $10M. This would allow mom & pop businesses to take advantage of lower capital gains rates while limiting the ability of the ultra rich to abuse the system.

Short-term, or long-term capital gains?

Yes they do know that. They are not stupid like the Republicans.

Not for much longer and for the record, the rates were much higher when the economy was booming and there was no debt/deficit.