The rationale I have heard for this is that a lower tax on capital gains is supposed to encourage investment
that's basic economics 101: everything else being equal, if you lower the tax (on anything), you encourage its production.
and increase the economic growth.
there are two distinct types of "capital gains": one from folks starting a business, investing in a new product, building a factory, aka, activities that yield real impact.
then there is investments in the secondary markets, buying and selling of ownerships in factories or products that the first kind produces. this type of investments creates a valuable incentive for the investors in the first kind so they do have an important role, but typically at much diminished levels.
to make matters worse, there are those people who work to produce such gains for other people. their "capital gains" are more like salaries, as a form of compensation for their work, not their putting their own capital at risk.
Unfortunately, people blends both kinds together when they talk about capital gains and taxes on capital gains, making the matter fuzzier for the average joe to comprehend.