Because people incur nominal debt contracts which causes wages to be sticky downwards --> they can go up more easily than down. With inflation, if wages are too high, you can let inflation eat away at them. If there were no inflation, the downward stickiness would lead to a lot of market failures (bankruptcies and unemployment) when wages were too high.
But then why do people incur nominal debt contracts? Because production occurs throughout time but must be settled in some medium. The farmer needs to know how much he will get for his wheat, the shipper needs to know, so someone needs to promise to buy wheat next year for $X and for that contract to be useful, everything else needs to be priced in terms of $ as well, so that the farmer can buy fertilizer for $ today, and agree to hire workers to work the field for $ from today to harvest time, etc. In this way, the farmer can estimate what he will get next year, what he will pay throughout the year, and what investments he needs to make right now. It is nothing special about the dollar -- if everyone used bitcoin for real contracts then wages would be downward sticky in terms of bitcoin, too. But as there is no way of expanding the supply of bitcoin, this downward stickiness would lead to a lot more market failures, and thus a lot more unemployment. We would then be effectively back on the gold standard, but at least with the gold standard, you'd occasionally find lots of gold in a mountain and this would stimulate the economy and increase employment. With BTC, you'd have all the financial crises of sticky prices but without the occasional gold discovery.
Why?