Promotion gets the turnstiles to spin and gets people into the park; but, after they are there - experience takes over. Even if an attraction earns its spot on the front of the park map for the month or even if it has FastPass, that doesn't translate into it earning the respect and popularity to pull guests to it and away from other attractions.
Stitch is a perfect example of this. It has decent capacity; but, the real reason for the lack of lines is that even in a park like the MK, even the casual tourist is confronted with the "re-ride" proposition. Determining which attraction they will visit again. Repeat business on attractions like Stitch just aren't deserving of the time investment on vacation (which your time on vacation is finite and the most valuable commodity). A five minute wait on Stitch isn't worth the 20 minutes spent total in the line, preshow, attraction. Guests would rather take their vaction hours elsewhere.
So, you can steer/crowd shape guests all you want from "popular" attractions to "under-utilized" assets all you want; but, when push comes to shove and you are staring into the lifeless abyss of your smartphone app or FP+ kiosk and you see a short wait time available for something like Stitch or Imagination, the value propostition of your investment of time is directly impacted by the quality of the attraction.
If you want to make lines for Space, Splash, Everest, Soarin', TSMM, Pan and the like shorter, the only way to do that is by adding in capacity. "New" always helps in that the shiny object gets all the attention; but, new also is different - which is what the historical guest patterns at the resort have already determined they want. They don't want to invest in Stitch. Either prune it by plussing the attraction or add something in worth the time investment elsewhere. The last thing that needs to take place is for the under-utilized attractions to go away. Even if guest ride it once and don't do so again, they are still helping. The parks are needing more. More in the form of new or more in the form of improved. The last several years have brought very little of either other than trying to change the way guests visit what is already there.
What certainly can't happen is exactly what has been happening - do as little as possible as you try to figure out a way to get guests to spend time in attractions they've already decided they don't want to wait in line for.