The primary motivations are that politicians in the US typically cater to those who are more well off (the donors) and those who vote (the elderly and the more wealthy). Studies show that actions of politicians in the US are generally not responsive to the interests of the lower income citizens in their district, but are highly correlated with the interests of the wealthy.
So who has a lot of capital gain and dividend income? The wealthy and elderly. If you have a diversified portfolio that provides $90k in dividends from the portion that's in your taxable account, you probably have $4.5 million PLUS whatever's in your IRA, and your house, etc.
If you're a middle-earning family, you probably don't have any substantial amount of funds in taxable accounts. But since huge numbers of people have 401ks with stocks in them, its easy to pretend like policies intended to help the wealthy donors are actually of benefit to the average joe--even though 401k dividends are not taxed, and the top 10% owns something like 50% of all the stocks.