The printed design has remained the same but the form factor has been pretty static for the last century or so. Things like bill readers are a big reason for that once they appeared it became a lot harder to swap around bill shapes.
(I know, I know, it's not actually that big a deal. The drawers have removable inserts, but you get the point of it.)
(actually, on second though, automatic bill readers in vending machines, etc. would need a big retrofitting, that's probably much more a big deal).