One reason is because the USD is currently the most traded currency right now. Some non-US countries accepts it and others peg their local currency to the USD. Radically changing the look and feel of it would cause a lot of unnecessary headaches; imagine you would have to educate everyone in the world that have any interactions with the new version so they don't think it's fake. Not to mention updating all the machines that are used to count/check them. Americans are also pretty conservative with change, the $1 BILL is still widely used despite the presence of the $1 coin, and people are still ok with the penny. (according to a 2014 poll, 43 percent said they would be “disappointed” or “angry” if the government stopped making them [pennies]. -NYT