Because a 18% tax on a poor, working single mother means she can't afford medicine or doctor visits. A 30% tax on someone making $1M or more per year doesn't. 

Because it's the sane and rational approach, having people pay according to their ability. The US used to have a 90% tax on the ultra wealthy and our economy was doing just fine. And today people like you want to make things "fair" by lowering it to historical lows to help the rich and further make tings tougher on the working poor. Additionally, you want to take away tax breaks for child care, student loan debt, and dependent credit for the poor (and everyone). This makes it even tougher for those at the bottom of the socio-economic ladder.

It's a horrible idea all the way around.