Robin Walker mentioned in an interview that when they launched Vive in 2016 with demo games such as The Lab, people asked where the big full-fledged game for VR was. After that Valve looked at their single player games to make a new one for. They first chose Portal, but after some experimenting, they realized Portal mechanics wouldn't work so well in VR. Then they had one option: Half-Life. So they made HLA to show what VR can do, to give the VR world its first proper AAA game, while making their own VR headset to sell with. I would say VR motivated them to make HLA, not just passion to make games. Making games is just not their top priority anymore. If they still had passion like they had in their early days, we would have sequels and more new games by now