It's inherent to the original film elements used by George Stevens and how he filmed the movie. It's the best we'll ever see with this particular movie. If you watched the Blu-ray or seen the movie on DVD or TV, the 4K disc improvements would be much more recognizable to you.
Listen to the linked podcast below. Sure, things have slowed down quite a bit and might continue to be the case for the immediate future. However, I think they will pick up again in time as WA is refilling the pipeline with titles they're working on, as indicated by Mr. Feltenstein's comments...
Just remember any HD title that is redeemed at MA, doesn't get that free iTunes 4K upgrade if it happens to come out later in 4K. The movie has to be redeemed at iTunes.