This is a very interesting quesion since when people sell bitcoin for USD, it means they trust USD more than bitcoin
So why do people trust in fiat money, which is created out of nothing by the central banks?
Because when you have $500 in cash, you know, that you can buy in one year a better notebook with that money than today. With 5BTC you don't know. People want to plan. They want stability.
The only reason the price of a notebook computer goes down year over year is because of advances in industrial production - be it efficiency or merely economy of scale. Also, there is a tremendous amount of competition in technology, which means that the company able to manufacture a quality component for the lowest cost is going to be preferred. So basically, the reason the cost of a notebook keeps going down is a direct benefit of a competitive free market situation.
That said, the purchasing power of the USD overall is going DOWN year over year, DESPITE increases in efficiency and industrial capacity. This is because the dollar is not backed by anything of tangible value, and thus can be printed out of nothing. There is absolutely no security in the value of the USD, but because it is associated with a country (and thus to the pride, nationalism and civic pride that goes along with it) people believe that the USD couldn't fail unless the US itself could fail (which of course it can't, so they believe).
So in short, there are two sides to the price question when it comes to economics: the first is the cost to produce a given product. The second is the value of money used to purchase that product. Since cost is stable (at any moment in time, the same amount of wealth is needed to purchase the same thing) the variable here is the value of money. Do you need $500 or 5BTC? Next year it will likely be $400 or 2BTC, based upon the fundamentals of the currency. But the truth remains - the purchasing power of the USD is very stable, and is decreasing at a very stable rate. The purchasing power of the BTC is much less stable, but is constantly increasing overall.
Hope that helps.