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.
What happened there?
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.