The US has the biggest financial market. The financial market of the US affects many other countries, it's like a domino effect. This will affect the bitcoin price. Other countries can ban Bitcoin today and it won't affect the price as much as when the United States bans bitcoins. It's like the US is at the centre of things when it comes to the financial market so when something happens to the US market it affects other markets to a certain degree.
Also, the US is one of the countries with the highest holders of bitcoins so what those people do with their coins might affect bitcoin price.