You're asking why they couldn't completely change the product from a full-fledged VR machine into a headset in roughly a year, especially when they already sell a regular VR headset for $1,000?
It was released in 2019 and by comparison, sucks. A new index with new displays of the frame but without the things that make the frame expensive (ram) would be welcome.
Valve Index Base Station: "Out of Stock"
You cannot buy a functioning Valve Index. And you wouldn't want one, because its 7 years out of date.