Don't they issue special edition coins every once in a while? Harriet Tubman was indeed an important but of history, but generally not as important to the era as, say, Lincoln.
Alternatively, we could regularly rotate historical figures on all our bills and coins. That would be another fair solution.
Flash forward thirty years, I guarantee you the $1, $5, $10 and $20 denominations will all have pictures of either MLK, Malcom X, Rosa Parks, Barack Obama and Harriet Tubman on them.
What makes you think that?