Disneyland had to re-do the entire flooring if I recall for the ride vehicles, or they had to switch out all their ride vehicles, it was one or the other.
My source tends to be vague, as I think they're afraid to share key information but from what I gathered talking prior about this and over the weekend about this they've told me (and this is quoted) "Ride's like this are hard to come by, which obviously mean they are hard to operate. Most rides have 1, 2, 3, maybe even 4 "steps" to it (Queue, pre show, load/unload, ride Ive assumed), this ride will have closer to 10 "steps" in total. If 1 "step", especially one of the very first ones, do not work, the ENTIRE ride must be shut down. It is very difficult to pull this all off under the circumstances the workers are under".
So I'm thinking this Dec 5 opening was a corporate (Iger) decision? Rushed opening for the Holidays is what it sounds like to me ultimately and they may pay the price for it. To my knowledge from reports on here and Magic, not my source, workers from Mickey and Minnie's were moved over to SWGE to make sure everything was done on time for the August opening, then like 90% of THOSE workers went on to help with Rise.
This may not really be news to people here in the know but I figured id share since theres not much "inside" info I tend to receive that is shareable. Sounds like there is A LOT of pressure to get this thing done.
TLDR; WDW Rise is figuring it all out from scratch, where DL fixed one problem, and has time to correct others while other said problem is currently being fixed. DL's was also built first.