According to a study reported at Science Daily, most currently-known U.S. presidents will probably fade from memory in another few decades.
A variety of reasons are suggested, many of which focus on the length of time from the date the particular president left office and the concurrent lack of people living who remember that president. Presidents who served before I was born, for example, don't fade away all that quickly for me because my parents and other older relatives remember them and so I can connect.
One possible flaw in the study is that it tests the memory of presidents in groups of college undergraduates. Given the large number of news reports that suggest that population's memory of history is not the strongest, we may wonder if these presidents will be as forgotten as the researchers think.
Another is that it overlooks a potential reason for forgetting certain presidents: The human brain will often "choose" to forget traumatic experiences. This has the advantage of also explaining the Democratic Party's strategy in the most recent midterm elections, in which candidates often seemed to forget who the current president was. Unfortunately for that strategy, many voters apparently remembered.