It's prob'ly because it feels incomplete... like it should be short for something else but not a name in and of itself... Have known plenty of people who get addressed as
Jo and some who have it as part of a double name (Billie-Jo), but it doesn't feel complete on its own...
JoleneJoannaJoanneJodiJosephineJoelle
They all feel more solid than just
Jo, but can use
Jo as a nn if they so choose. I'm partial to
Kendra Jolene nn Kendra-Jo. I believe "Designing Women" had a Mary-Jo. DJ on "Full House" was short for Donna-Jo. However,
Jo on its own is missing something.