I don't necessarily know if it's a "lack" of C/D tickets. If you look in recent years, we've seen things like/are getting Pets, Yoshi, Fallon, Swirling Saucers, Bourne, Na'vi River Journey, etc. There's been other stuff but not enough. I would consider MMRR and Rat a D, but I believe the true definition of an E-ticket from the beginning are the rides that are some of the most popular (which is why they were more expensive), which would technically classify them as an E.
So what I just named off between Universal and Disney isn't actually a lot, you're right. However, I think most of the problem stems from a shift in guest expectations expecting every new ride to be an E-ticket. Disney saw WWoHP and said they wanted something like that. So they've been trying and trying ever sense and they don't seem to get that Potter works because it's straight out of the books and movies. This is a place fans want to go and have an emotional attachment to. The same can be said for SNW - there's a strong emotional attachment there and from everything i've seen, the land looks just like stepping into a game.
No one has an emotional attachment to Batuu, Avengers Campus, Toy Story Land, or Pandora. In those four lands, there's a combined 9 rides and the best one is next to impossible to get on since you need a ride reservation. There's also only one ride out of those 9 that doesn't have a height requirement (yes, Swirling Saucers has as height requirement).