Comes down to -- as you would expect with a Disney park -- a mixture of theming and practicality.
When Epcot was built, the initial conceit was that Future World would maintain the MK "model" while World Showcase was supposed to be a "world's fair" kind of place where people of all different cultures would mingle. Hence the tunnels under FW and the "backstage / onstage" set-up for Epcot.
Basically, Walt's thought was that you didn't have to "hide" World Showcase cast, because World Showcase was "real" as opposed to "fantasy".
The whole point of the Studios when it was built was to be a working Studio, where Guests would regularly see production or operations cast moving from place to place and get a glimpse of what was going on "backstage". They wanted people to see Characters riding in carts to their next "location" and to see people in costume coming and going. So, again ... no tunnels or underground service areas needed.
With DAK, it would be somewhat impractical to have everything underground, since so much of what that park deals with is animal-based.