Value is a function of supply and demand. A classic question in economics is why water is cheap, when it is essential to life, and diamonds are expensive, even though they just look pretty (of course, that doesn't hold totally nowadays, but the question is centuries old). The standard answer is that while people want both water and diamonds, there's a lot of water, but not a lot of diamonds. These basic forces also govern the prices of most things.
Well, US dollars have an incredibly high demand. The first part comes from how a couple TRILLION dollars are needed a year to pay taxes. The government won't take anything else for taxes, and will punish you if you don't pay what you owe, so that's a massive source of demand. The other is that US dollars are legal tender, and while that doesn't matter as much anymore, for a very long time most payments were made in cash, and if you received a service by law you could pay in USD and the merchants couldn't refuse it. That's still true but secondary, however the financial system is designed for it (in the USA of course) in large part because of that. So USD have intrinsic value because people NEED them to pay taxes, whereas bitcoins aren't needed for anything, and can't compare with a couple trillion dollars' worth of guaranteed demand.