My guess is because how big advantage amazon has compared to smaller companies. They have good tax cut. Taking that aside, they also applied tight working time and high kpi, which can't be applied by smaller companies. And it gets better with PR stunt like this.

And CMIIW, I think increased minimum wage will increase goods price and higher skilled worker's wage overall, making those changes useless again.

It's just my 2 cents, with source limited from following recent news.