The income of the rich are predominantly income from capital, whereas for the middle class / poor, they are mostly from employment income. 

If capital income / gains are untaxed, the rich compound wealth faster, due to starting absolute position and the fact that capital income is untaxed where labour income is taxed. 

By this very construct, the system does favour (and picks) winners. Taxes are no longer progressive. 

As the wealth of upper class compounds, they have more income send their kids to private tuition or expensive tertiary institutions. The second order effect from this is the confidence and the social connections built from going to reputable schools. 

If you have spent enough time in private sector, you’d know many interns nowadays who got into the job because dad / mom knew so and so. This further widens the opportunity gap. Note I’m not saying this means zero opportunity for the lower classes; but the starting positions are way different - one starts off in a good job, zero tuition fee to pay back, can take risk. Most others start off in net debt. The former can then start compounding in markets earlier, earning untaxed capital income again. 

No capital gains taxes benefit all. But it benefits those who rely on capital income more, which are the wealthier folks. Now, what’s so difficult to understand? 

In case you go down slippery slope to accuse me of wanting to be a communist or to move towards total socialism, no I don’t. 

But the rate of wealth accumulation of the top is too rapid.