Performance issues and engine difficulties were too much to overcome.
Minecraft Super Duper Graphics Pack

Sadly, Mojang announced today that they are cancelling the promised Super Duper Graphics Pack that was originally revealed at E3 2017. This visual overhaul was teased for the game during the annual industry trade show and fans were eager to check it out for themselves.

This was going to be an optional piece of DLC that brought a suite of visual improvements to the Xbox One and PC versions of the game without needing to mod it. Sure, a lot of the visual improvements have been done time and time again on the PC through a huge number of mods, but there was never anything official. This aimed to change that. Some of the improvements included full 4K support, HDR, dynamic shadows, foliage that moved in the wind, vastly improved shader work, God rays streaming through fluffy clouds, and the like. Everything would still be made out of blocks, but those blocks looked really damn good!

The project just proved to be too difficult for the team to properly implement.

Roughly one year ago, Mojang's Tamer Jeison provided a small update about the pack's development. He notes that a major hurdle came with the engine that the team has been using. It led to poor performance even on the Xbox One X.

The Graphics Team is still hard at work on this (it ended up being a ton more work than initially anticipated). The graphics part of our engine was quite honestly simply not built for this, and when we tried to put all the new features in piece by piece things went bad. Performance, even on the Xbox One X, was between bad and inconsistent during all the internal play sessions we had.
The team even spent a decent amount of time rebuilding parts of their engine to work with the graphical overhaul. Sadly, it was to no avail. Here is the official word on the death of the Super Duper Graphics Pack.

Some of you might remember us announcing the Super Duper Graphics Pack during E3 2017. Super Duper was an ambitious initiative that brought a new look to Minecraft but, unfortunately, the pack proved too technically demanding to implement as planned.

We realise this is disappointing to some of you – there was a lot of enthusiasm for Super Duper from inside and outside the studio – but unfortunately, we aren’t happy with how the pack performed across devices. For this reason, we're stopping development on the pack, and looking into other ways for you to experience Minecraft with a new look.

Minecraft is available on a wide range of platforms, and we’re constantly trying to make the most of the technical architecture of each one. We’ll be able to share more on that subject very soon.

Thanks for your patience, and sorry for not following through on our plans!
Maybe we'll see another go at the Super Duper Graphics Pack when the new Xbox is released. If you don't want to wait for that possibility, you can always try your hand at custom modding the PC versions of the game. There are a lot of options available to you, including the SEUS PTGI shader that implements ray traced lighting from Sonic Ether.