1) the screens are really close to your eyes, which is an issue for your eyes to focus on
2) the screens need a high field of view
3) the headset need to provide 6D pose feedback to the computer
4) battery
Resolution. VR screens don't yet have small enough pixels to use them for typical tasks like word processing, spreadsheets, and code development. It's hard to cram 4000 horizontal pixels into a 1" wide screen.
Is it the shape of the screen?