Half Life was always about pushing the boundaries of gaming. The first Half Life with their combination of story telling in a 3D shooter environment was absolutely at the sharp end of the field at that time. If you’ve seen the Black Mesa documentary you’ll know why HL2 was such a hit and how it was revolutionary at that time. After that they did some DLC, but Valve wasn’t happy with what they were doing. It wasn’t groundbreaking, it was just creating content for the sake of content. As they didn’t need any more money from creating games, they opted to not create HL3. It wasn’t till VR became more mainstream they again tried to do something at the sharp end of the field, by creating HL Alyx.
I don’t know what would prompt them to ever make a HL3 if such a thing even exists.