Well, it happened because Valve used to consistently make high quality games, and Steam used to actually be a good service. It stopped because Valve just kinda stopped making games around DOTA 2, and it really stopped when Half-Life 2's writer posted a "fanfic" that was blatantly just what the storyline of Half-Life 2: Episode 3 was supposed to be, which caused many people to see that as the final nail in the coffin for any hope of Half-Life 3.
Why did it happen or why did it stop?
Well, it happened because Valve used to consistently make high quality games, and Steam used to actually be a good service. It stopped because Valve just kinda stopped making games around DOTA 2, and it really stopped when Half-Life 2's writer posted a "fanfic" that was blatantly just what the storyline of Half-Life 2: Episode 3 was supposed to be, which caused many people to see that as the final nail in the coffin for any hope of Half-Life 3.