Why do you think congress has established different maximum tax rates on capital gains of individuals?

I think the basic reason behind it is they have to consider the ability of individuals to sustain their living after paying taxes.

At certain level of earning, some people still have a lot of capital left after paying their taxes (which will not affect their living / lifestyle), meanwhile this won't be the case if you're seeing the individuals in lower level of earning. So they thought is it okay to tax the rich a little bit higher.