What do you think is most likely to be taking them so long?

Hardware/software/other technical aspects:

• keeping their software in sync with new hardware - obviously (?) it’s going to be a 64-bit game, but maybe there are other aspects to it, for instance, adapting the maximum quality of textures, meshes, shaders, animations and what have you to be appropriate for today’s computing power (CPU, GPU, new PCIe standards, etc), considering how long ago the game’s development began.
• game engine - are they making something from scratch, reusing older assets or “borrow” for instance UE5?
• other?

World building aspects:

• there are tons of loose ends to tie up or to build upon from previous games - did they not have plans for a sixth game when Skyrim released and thus were left clueless on how to develope the story? Most unlikely…
• are they taking time trying to incorporate fan theories and stories?
• resolving contradictions from previous games?
• other?

Political aspects:

• the acquisition by Microsoft?
• internal strife?
• budget?
• other?

Other aspects:

• “simply” fine tuning to achieve perfection?
• all of the above?
• none of the above?

Context: I have never ever participated in game development ✌️