Uncle Sam is has been the common national personification of the American government since the War of 1812. He is old, grumpy and has bad taste in clothes. He is no longer fit to represent America as a national symbol.
Captain American on the other hand is awesome. He represents the essence of America. Captain American is indestructible, kindhearted, stands up for what is right and has a ultimate patriotic outfit. He is the "Super American."
To change the national personification of America from Uncle Sam to Captain America would increase patriotism and achieve an overall sense of unity and national pride.