The first lady thing I think has to do with the fact that the first lady has had a history of actually doing stuff in US politics.
It's like electing a pair to be president almost. In many cases the Presidents wife does more then the Vice President.
They don't always get involved, but they can and often do, mostly in charitable work... and sometimes even play a bigger role
Edith Bolling Galt Wilson was practically president for a while when woodrow wilson was sick.
Elenaor Roosevelt was a huge impact... on social issues and tried to keep FDR in check during japanese internment and some of his other civil rights violations.
Hillary Clinton took it to a whole nother level. Her being first lady is a big reason she's a serious presidential contender.
