The US currency has remained generally the same for many years. Though the designs differ slightly over time to accommodate for counterfeit protection, the green color and classic images have remained for a long time. As we move farther in the 21st Century, it is time to reconsider the viability of our current currency. When looking at foreign currency, many of them feature modern designs that receive thorough updates every few years.
By updating our designs, the government praise the innovations of recent years and accommodate those who are visually impaired. Many things go into currency design. One such example can be found here: http://www.travispurrington.com/211378/2317660/gallery/2014-usd-proposal