There is also a VR mod on Steam that works perfectly With Half-Life 2, No need to jump through hoops, it just works. One of the best VR experiences out there.
That's the difference; does the mod allow interaction with the world like Alyx, or is it just you're there, but still using basic 2D controls to manipulate shit (press E to use a door instead of grabbing the handle and pushing/pulling)?
I'm not sure how the game engine can incorporate such fine detailed mechanics. It almost seems like it'll have to be reworked from the ground up, with only the assets being maintained.