James K. Polk is a very interesting case, as his actions and policies as a president had a significant impact on American history, but he is not remembered in popular culture. As another user pointed out, he is often an important figure in the eyes of historians due to his ability to carry out the policies he campaigned about, but yet he is almost entirely forgotten by the general public.

This is likely due to two major factors; the length of time from his presidency to the modern day, and the fact that historically presidents are usually not remembered entirely for their policies. To expand on the latter point, presidents from around Polk's time who are remembered are ones such as Washington, Jefferson, Jackson, and Lincoln. These presidents are known only partly for their political accomplishments, but also in conjunction with one or more major factors, such as founding the USA, being assassinated, having a fascinating personal character, presiding over a war in which the future of the USA was in question, or being featured in modern day monuments and/or currency.

Polk suffers from the fact that his legacy is not well defined outside of the four years he spent in office. He was not a man who was the subject of American mythology like Washington or Jackson, and didn't have a relatively noteworthy character. He died just over 100 days after leaving office at 53 years old, and did not have a later career to expand upon, while other presidents are remembered for events that occurred outside their actual presidential terms. He also only served one term which he narrowly won, and was not particularly exalted by the major political parties, which often can serve as a major vehicle for advancing a president's legacy.

Lastly, the length of time is a major factor. Studies have suggested that many recent presidents who still reside in the public consciousness will also disappear due to time. A modern comparison to Polk would be Lyndon B. Johnson, a man who was responsible for major policies in American history in regards to Civil Rights and the Vietnam War. Without a truly defining characteristic, time tends to fade the public memory of men such as Polk.

Edit: spelling