It's just because I enjoy the HL universe way more than I do the Sin one. It seems many gamers felt the same.
I think a very important part is that there ISN'T much of a Sin Universe to enjoy. Looking back at the time when I played Sin and HL, I realize it has been pretty different back then. After HL I didn't really want to know how the story continued, that only came with the HL2 hype. Meanwhile Sin made me want to play on.
HL set up a story with the original game, explored the catastrophe at Black Mesa from two more angles in expansions, disappeared for a few years and came back strong with HL2 and then the first episode to continue the story. HL2 added a lot of substance to the HL "universe", and because of the rather subtle storytelling, people want more episodes/games to play.
Sin on the other hand does not have this game that adds significantly more story to become a "universe" because Emergence is so far only "more of the same". It's not an universe at all, although it could have been. Everything is focused on "chase Elexis" during the first game, the expansion added a more or less independent plot with a twist. Everything evolves around Elexis, about how she disappears and about if she's coming back. With only Emergence, nothing makes sense anymore. Elexis is back, period. How? Why? When? No idea, I prolly have to play 1337 episodes to find out (this post might imply I don't like Emergence but I enjoyed it quite a bit, and I really want them to finish what they had in mind with the story). Freeport is a completely different city now, same questions, no answers here. There's the "mysterious" sidekick - but in fact she's just rather annoying and doesn't contribute much besides being a meatshield for the rather unfair end levels. There we have a confused and mute Blade and we have no idea WTF he's up to. I wanted to know more about what happened after the first game but now I hardly care anymore. "Throw player into new world and slowly uncover the changes since the last game" simply doesn't work in an episodic sequel to a several-years old game without some people losing interest and others not being interested to begin with.
IMO they tried to mimic Valve's approach and failed because there was no "HL2" version of Sin. And to be honest, would we actually be as psyched about HL2 if after 6 years there was nothing but a 4hr episode with a story that seems to make no sense, explains nothing and offers little new? Say, if instead of HL2 we would had gotten the part from waking up in the train till reaching Dr Vance, with only the crowbar, pistol and SMG for weapons and nothing but the initial combine troopers and headcrabs as enemies, plowing through some backyards and then an extended sewer sequence - what would we have thought of the game (also ignoring that the fanbase of HL is a lot larger to begin with)? Sin needed a whole game to introduce the player to the new Freeport and the returned Elexis, uncover parts of the story development and THEN it could have jumped to episodes, explaining what was going on in detail.
Besides, although partially fun Emergence suffered from several bad design decisions. The limited variety overall (few badguys to fight, only three guns, scenery) and the overall look and feel more often than not made me think I'm playing a watered down version of HL2, cut to sell in several parts, instead of a unique game. Add to that after Sin vs. HL, they should have known that launching Emergence only shortly before the much more hyped/coveted HL2-Ep1 was going to be a bad idea. It all came down to Emergence being directly compared to "Aftermath" (which to me felt more like a short expansion than a real "Episode" with the features of the original FULL game + some) and falling short of the latter in almost every aspect.
{ There is no such thing as innocence, only degrees of guilt }
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