https://www.washingtonpost.com/news/the-fix/wp/2015/03/06/why-is-andrew-jackson-on-the-20-bill-the-answer-may-be-lost-to-history/
My personal theory? Jackson was a southerner who was pro union and spoke for preserving the Union. The South was on its way to being re-integrated in American society again and we had a Democrat in the White House a decade earlier. It was probably seen as a gesture to Southerners. Just my pet theory.
Americans always get freaked out when the money design changes - overseas money design changes frequently and no one gets worked up over it. It happens in the USA and suddenly we are bombarded with conspiracy theory freak outs. It is no big deal. In fact I would love to see the faces on the money rotated out every decade. Ronald Reagan on a bill for a decade followed by a new presidential face on a bill for another decade, etc.
But would we want a Bill Clinton dollar?