The suit he is wearing denotes that he is a historical person and the period when he had lived.
George Washington (1732-1799), Charles Willson Peale, 1784, From the collection of: Harvard Art Museums
It is not appropriate for banknotes because it does not seems like a real person. Also, his smiling face shows friendliness but it does not match with banknotes which require the formality.
This user gallery has been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.