I honestly think Disney should be concern with improving the food at DHS than increasing food options at World showcase. I am sure that there are people that go to DHS after eating dinner outside of DHS like I do. ABC Commissary and Hollywood & vine have bad reputations. 50's prime time cafe isn't for everyone if you don't want to deal with the banter over there.
Except that large swaths of the WS dining is run by 3rd parties. That I assume take on a large chunk of the cash outlay involved in any new construction/renovations as part of their deal. Morocco happens to be one such area run by a 3rd party I believe. I don't know the corporate name, but see here -> http://restaurantmarrakesh.com/mar/about_us.asp
If you look at the major restaurant renovations/change in Epcot dining in the last few years, they all seem to be focused on the 3rd party owned/operated venues. New restaurant in Mexico? 3rd party (Palmas Services LLC). Changes to France dining? 3rd party (JBI LLC) Refurb of Yakitori House? 3rd Party (Mitsukoshi USA) If you look at the Disney O/O locations, it's been mostly maybe a small menu change, or the addition of a new beer cart nearby.
Judging from WDWs current perceived lack of care about the overall quality of food, I'm not terribly surprised that DHS lingers. But as you said, people eat dinner outside of DHS. If you spend the time going to another on property restaurant, that is time spent not on a ride or in line, helping them to not need to add capacity, and they get your money anyway. Leaving them minimal reasons to change. The 3 non sig TS locations are usually booked solid, they seem to be content with Brown Derby lingering near the bottom of the sig food chain, and they need a buffet in park for character meals, so H&V won't change much as it still does solid business w/characters.